[libcxx] Remove XFAILs for older macOS versions

That test doesn't fail anymore since r349378, since the assertions that
r349378 removed must have been bugs in the dylib at some point.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@349484 91177308-0d34-0410-b5e6-96231b3b80d8