stbt.FrameObject base class: Remove @for_object_repository decorator
stbt.FrameObject base class: Remove @for_object_repository decorator

This decorator is used for marking indirect subclasses of FrameObject,
that wouldn't be otherwise detected by Stb-tester's [Object Repository].
It doesn't make any sense to apply it to the FrameObject base class
itself.

[Object Repository]: https://stb-tester.com/manual/object-repository

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester