Remove unused code

src/main/java/io/github/thebusybiscuit/slimefun4/api/items/SlimefunItem.java

@@ -31,7 +31,6 @@
 import io.github.thebusybiscuit.slimefun4.api.exceptions.IncompatibleItemHandlerException;
 import io.github.thebusybiscuit.slimefun4.api.exceptions.MissingDependencyException;
 import io.github.thebusybiscuit.slimefun4.api.exceptions.UnregisteredItemException;
-import io.github.thebusybiscuit.slimefun4.api.exceptions.WrongItemStackException;
 import io.github.thebusybiscuit.slimefun4.api.player.PlayerProfile;
 import io.github.thebusybiscuit.slimefun4.api.recipes.RecipeType;
 import io.github.thebusybiscuit.slimefun4.api.researches.Research;
@@ -575,20 +574,6 @@ private void loadItemHandlers() {
         }
     }
 
-    /**
-     * This method returns whether the original {@link SlimefunItemStack} of this
-     * {@link SlimefunItem} is immutable.
-     * 
-     * If <code>true</code> is returned, then any changes to the original {@link SlimefunItemStack}
-     * will be rejected with a {@link WrongItemStackException}.
-     * This ensures integrity so developers don't accidentally damage the wrong {@link ItemStack}.
-     * 
-     * @return Whether the original {@link SlimefunItemStack} is immutable.
-     */
-    protected boolean isItemStackImmutable() {
-        return true;
-    }
-
     /**
      * This method checks if the dependencies have been set up correctly.
      * 

src/main/java/io/github/thebusybiscuit/slimefun4/api/items/SlimefunItemStack.java

@@ -42,7 +42,6 @@
 import io.github.bakedlibs.dough.skins.PlayerSkin;
 import io.github.thebusybiscuit.slimefun4.api.MinecraftVersion;
 import io.github.thebusybiscuit.slimefun4.api.exceptions.PrematureCodeException;
-import io.github.thebusybiscuit.slimefun4.api.exceptions.WrongItemStackException;
 import io.github.thebusybiscuit.slimefun4.implementation.Slimefun;
 import io.github.thebusybiscuit.slimefun4.utils.HeadTexture;
 import io.github.thebusybiscuit.slimefun4.utils.compatibility.VersionedItemFlag;