Revert "[OPENMP] Fix PR38026: Link -latomic when -fopenmp is used."

This reverts commit r336467: libatomic is not available on all Linux
systems and this commit completely breaks OpenMP on them, even if there
are no atomic operations or all of them can be lowered to hardware
instructions.

See http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20180716/234816.html
for post-commit discussion.

llvm-svn: 337722