[libc++] Removes deprecated _LIBCPP_ENABLE_<VERSION>_REMOVED_FEATURES macros (#88548)

We marked those macros as deprecated in the last release with the intent
of
removing them in LLVM 19. This commit performs the removal.

GitOrigin-RevId: 41a830500aa5556a65198607ec751d8e3254c949
6 files changed