[libcxx] Remove the locale fallback for NuttX

Since these functions can handled by NuttX's libc now

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D108895

NOKEYCHECK=True
GitOrigin-RevId: 3285c7a4364eb88fd6753f362d74ee008d5479de
3 files changed