#1272: Only allow one entity in CraftItem instead of two

This pr reverts a change made in 2012 which allowed CraftItem to be a also other entities such as an arrow.
I don't see any practical reason why the original change was done, other than to allow plugin developers to do some stuff in a cursed way.
With the introduction of the PlayerPickupArrowEvent in 2016 there is also a way to access the arrow involved in a better way.
Generally I think the current behavior is a bit strange and should be reverted.

nms-patches/net/minecraft/world/entity/projectile/EntityArrow.patch

@@ -46,7 +46,7 @@
 +            ItemStack itemstack = this.getPickupItem();
 +            if (this.pickup == PickupStatus.ALLOWED && !itemstack.isEmpty() && entityhuman.getInventory().canHold(itemstack) > 0) {
 +                EntityItem item = new EntityItem(this.level(), this.getX(), this.getY(), this.getZ(), itemstack);
-+                PlayerPickupArrowEvent event = new PlayerPickupArrowEvent((org.bukkit.entity.Player) entityhuman.getBukkitEntity(), new org.bukkit.craftbukkit.entity.CraftItem(this.level().getCraftServer(), this, item), (org.bukkit.entity.AbstractArrow) this.getBukkitEntity());
++                PlayerPickupArrowEvent event = new PlayerPickupArrowEvent((org.bukkit.entity.Player) entityhuman.getBukkitEntity(), new org.bukkit.craftbukkit.entity.CraftItem(this.level().getCraftServer(), item), (org.bukkit.entity.AbstractArrow) this.getBukkitEntity());
 +                // event.setCancelled(!entityhuman.canPickUpLoot); TODO
 +                this.level().getCraftServer().getPluginManager().callEvent(event);
 +

src/main/java/org/bukkit/craftbukkit/entity/CraftItem.java

@@ -1,7 +1,6 @@
 package org.bukkit.craftbukkit.entity;
 
 import java.util.UUID;
-import net.minecraft.world.entity.Entity;
 import net.minecraft.world.entity.item.EntityItem;
 import org.bukkit.craftbukkit.CraftServer;
 import org.bukkit.craftbukkit.inventory.CraftItemStack;
@@ -9,50 +8,49 @@ import org.bukkit.entity.Item;
 import org.bukkit.inventory.ItemStack;
 
 public class CraftItem extends CraftEntity implements Item {
-    private final EntityItem item;
-
-    public CraftItem(CraftServer server, Entity entity, EntityItem item) {
-        super(server, entity);
-        this.item = item;
-    }
 
     public CraftItem(CraftServer server, EntityItem entity) {
-        this(server, entity, entity);
+        super(server, entity);
+    }
+
+    @Override
+    public EntityItem getHandle() {
+        return (EntityItem) entity;
     }
 
     @Override
     public ItemStack getItemStack() {
-        return CraftItemStack.asCraftMirror(item.getItem());
+        return CraftItemStack.asCraftMirror(getHandle().getItem());
     }
 
     @Override
     public void setItemStack(ItemStack stack) {
-        item.setItem(CraftItemStack.asNMSCopy(stack));
+        getHandle().setItem(CraftItemStack.asNMSCopy(stack));
     }
 
     @Override
     public int getPickupDelay() {
-        return item.pickupDelay;
+        return getHandle().pickupDelay;
     }
 
     @Override
     public void setPickupDelay(int delay) {
-        item.pickupDelay = Math.min(delay, Short.MAX_VALUE);
+        getHandle().pickupDelay = Math.min(delay, Short.MAX_VALUE);
     }
 
     @Override
     public void setUnlimitedLifetime(boolean unlimited) {
         if (unlimited) {
             // See EntityItem#INFINITE_LIFETIME
-            item.age = Short.MIN_VALUE;
+            getHandle().age = Short.MIN_VALUE;
         } else {
-            item.age = getTicksLived();
+            getHandle().age = getTicksLived();
         }
     }
 
     @Override
     public boolean isUnlimitedLifetime() {
-        return item.age == Short.MIN_VALUE;
+        return getHandle().age == Short.MIN_VALUE;
     }
 
     @Override
@@ -61,28 +59,28 @@ public class CraftItem extends CraftEntity implements Item {
 
         // Second field for EntityItem (don't set if lifetime is unlimited)
         if (!isUnlimitedLifetime()) {
-            item.age = value;
+            getHandle().age = value;
         }
     }
 
     @Override
     public void setOwner(UUID uuid) {
-        item.setTarget(uuid);
+        getHandle().setTarget(uuid);
     }
 
     @Override
     public UUID getOwner() {
-        return item.target;
+        return getHandle().target;
     }
 
     @Override
     public void setThrower(UUID uuid) {
-        item.setThrower(uuid);
+        getHandle().setThrower(uuid);
     }
 
     @Override
     public UUID getThrower() {
-        return item.thrower;
+        return getHandle().thrower;
     }
 
     @Override