refactor: move `unregister` to common API module

bukkit/src/main/java/net/william278/husksync/api/BukkitHuskSyncAPI.java

@@ -76,14 +76,6 @@ public class BukkitHuskSyncAPI extends HuskSyncAPI {
         instance = new BukkitHuskSyncAPI(plugin);
     }
 
-    /**
-     * <b>(Internal use only)</b> - Unregister the API for this platform.
-     */
-    @ApiStatus.Internal
-    public static void unregister() {
-        instance = null;
-    }
-
     /**
      * Returns a {@link OnlineUser} instance for the given bukkit {@link Player}.
      *

common/src/main/java/net/william278/husksync/api/HuskSyncAPI.java

@@ -77,6 +77,14 @@ public class HuskSyncAPI {
         return instance;
     }
 
+    /**
+     * <b>(Internal use only)</b> - Unregister the API for this platform.
+     */
+    @ApiStatus.Internal
+    public static void unregister() {
+        instance = null;
+    }
+
     /**
      * Get a {@link User} by their UUID
      *