[libc++] Remove empty header xlocale/xlocale.h

Summary:
I can't think of a reason for shipping this empty header. If there is
a reason to do so, then hopefully this review can uncover it.

Reviewers: mclow.lists, EricWF

Subscribers: mgorny, christof, jkorous, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D59137

llvm-svn: 355829