Minor code style fixups

- Add missing @Override annotations. I can't find a way to enforce this
   with checkstyle - maybe I need spotbugs too D:.
 - Remove superflous "this"es.

src/main/java/dan200/computercraft/shared/util/FakeNetHandler.java

@@ -324,7 +324,7 @@ public class FakeNetHandler extends ServerPlayNetHandler
         @Override
         public void disconnect( @Nonnull ITextComponent message )
         {
-            this.closeReason = message;
+            closeReason = message;
         }
 
         @Override