Shortcut annotations `@QueryMapping`, `@MutationMapping`, and `@SubscriptionMapping` should target ElementType.METHOD only

[rstoyanchev]

The Javadoc says you can use these both at the method and at the class level, in which case all handlers methods in the controllers inherit that type name, but that's pointless currently since a method still needs to be annotated, e.g. with the shortcut annotation, so that doesn't save anything.

[rstoyanchev]

It looks like this came from #195 but I think that should be covered by a class-level @SchemaMapping. I can't see a good reason for shortcut annotations on the class level as you would still need to annotate individual handler methods and for that the shortcut provides the same.