Use @NotNull in methods rather than @Nullable

Separate safe external methods from internal only methods
Remove unnecessary methods

src/main/java/com/gmail/nossr50/mcMMO.java

@@ -345,7 +345,6 @@ public class mcMMO extends JavaPlugin {
 
             formulaManager.saveFormula();
             holidayManager.saveAnniversaryFiles();
-            placeStore.cleanUp();       // Cleanup empty metadata stores
             placeStore.closeAll();
         }