[libc++] Remove workarounds for systems that used to require __need_XXX macros

Libc++ tried accomodating systems that need to be able to define various
__need_FOO macros before including C library headers, however it does not
appear to be needed anymore in most cases. Indeed, glibc used to use that
system to conditionally provide definitions, however almost all instances
of these macros have been removed from glibc years ago.

I think the next step would be to also fix Clang's own builtin headers
to stop needing these macros.

Differential Revision: https://reviews.llvm.org/D131425