[libcxx] Remove unused macro _LIBCPP_HAS_UNIQUE_TYPEINFO

Summary:
We already have the negation of that as _LIBCPP_HAS_NONUNIQUE_TYPEINFO.
Having both defined is confusing, since only one of them is used.

Reviewers: EricWF, mclow.lists

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D54537

llvm-svn: 349947