Clean up turtle/pocket computer item creation

- Remove ITurtleItem (and ITurtleBlockEntity): this was, AFAIK, mostly
   a relic of the pre-1.13 code where we had multiple turtle items.

   I do like the theory of abstracting everything out behind an
   interface, but given there's only one concrete implementation, I'm
   not convinced it's worth it right now.

 - Remove TurtleItemFactory/PocketComputerItemFactory: we now prefer
   calling the instance .create(...) method where we have the item
   available (for instance upgrade recipes).

   In the cases we don't (creating an item the first time round), we now
   move the static .create(...) method to the actual item class.

projects/common/src/main/java/dan200/computercraft/shared/turtle/blocks/TurtleBlockEntity.java

@@ -39,7 +39,7 @@ import net.minecraft.world.phys.Vec3;
 import javax.annotation.Nullable;
 import java.util.Collections;
 
-public class TurtleBlockEntity extends AbstractComputerBlockEntity implements BasicContainer, ITurtleBlockEntity {
+public class TurtleBlockEntity extends AbstractComputerBlockEntity implements BasicContainer {
     public static final int INVENTORY_SIZE = 16;
     public static final int INVENTORY_WIDTH = 4;
     public static final int INVENTORY_HEIGHT = 4;
@@ -189,39 +189,30 @@ public class TurtleBlockEntity extends AbstractComputerBlockEntity implements Ba
         onTileEntityChange();
     }
 
-    // ITurtleTile
-
-    @Override
     public @Nullable ITurtleUpgrade getUpgrade(TurtleSide side) {
         return brain.getUpgrade(side);
     }
 
-    @Override
     public int getColour() {
         return brain.getColour();
     }
 
-    @Override
     public @Nullable ResourceLocation getOverlay() {
         return brain.getOverlay();
     }
 
-    @Override
     public ITurtleAccess getAccess() {
         return brain;
     }
 
-    @Override
     public Vec3 getRenderOffset(float f) {
         return brain.getRenderOffset(f);
     }
 
-    @Override
     public float getRenderYaw(float f) {
         return brain.getVisualYaw(f);
     }
 
-    @Override
     public float getToolRenderAngle(TurtleSide side, float f) {
         return brain.getToolRenderAngle(side, f);
     }