[libc++] Remove libc++'s own <limits.h> (#65472)

We do not define anything beyond the C Library's limits.h, so it
shouldn't be needed. This removes the need for a GCC-specific
workaround, see [1].

[1]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107795

Differential Revision: https://reviews.llvm.org/D138384