Remove unused ShouldFoldAtomicFences flag.

I think it's almost impossible to fold atomic fences profitably under
LLVM/C++11 semantics. As a result, this is now unused and just
cluttering up the target interface.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179940 91177308-0d34-0410-b5e6-96231b3b80d8