[GNU ObjC] Fix a regression listing methods twice.

Methods synthesized from declared properties were being added to the
method lists twice.  This came from the change to list them in the
class's method list, which missed removing the place in CGObjCGNU that
added them again.

Reviewed By: lanza

Differential Revision: https://reviews.llvm.org/D91874

(cherry picked from commit d1ed6703)