Revert "Fixed backwards compatibility"

This reverts commit 1f7cf78491.

eco-api/src/main/java/com/willfp/eco/core/gui/slot/FillerSlot.java

@@ -1,10 +1,8 @@
 package com.willfp.eco.core.gui.slot;
 
-import com.willfp.eco.core.items.TestableItem;
 import org.bukkit.entity.Player;
 import org.bukkit.inventory.ItemStack;
 import org.jetbrains.annotations.NotNull;
-import org.jetbrains.annotations.Nullable;
 
 /**
  * A filler slot is a slot that does nothing when clicked.

eco-api/src/main/java/com/willfp/eco/core/gui/slot/Slot.java

@@ -36,10 +36,7 @@ public interface Slot {
      * @param player The player.
      * @return The item, or null if no captive default was found.
      */
-    @Nullable
-    default TestableItem getCaptiveDefault(@NotNull Player player) {
-        return null;
-    }
+    @Nullable TestableItem getCaptiveDefault(@NotNull Player player);
 
     /**
      * Create a builder for an ItemStack.