[libcxx] Remove XFAILs for older macOS versions
[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: 756ef344af921d95d562d9e9f9389127a89a6314@349484 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx