[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 d1ed67037de6f3f44dc446784f74f0e02adec9b5)