Remove unused _LIBCPP_RAW_ITERATORS

Summary: This change removes seemingly unused _LIBCPP_RAW_ITERATORS.

Reviewers: #libc, EricWF, ldionne

Reviewed By: #libc, EricWF, ldionne

Subscribers: dexonsmith, ldionne, gribozavr2, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D79323