Removed OSX clang build.
Removed OSX clang build.

Clang and gcc are both clang at OSX. Instead of testing the same compiler twice,
we'll test against different versions of xcode now.

RWTH-HPC/PnMPI,RWTH-ELP/PnMPI,RWTH-ELP/PnMPI,RWTH-ELP/PnMPI,RWTH-HPC/PnMPI,RWTH-HPC/PnMPI