[libc++] Remove _LIBCPP_C_HAS_NO_GETS (#77346)

Since we use _LIBCPP_USING_IF_EXISTS to handle missing C library functions
now, _LIBCPP_C_HAS_NO_GETS shouldn't be necessary anymore.
See the discussion thread in #77242 for more details.

GitOrigin-RevId: 761b9d9e4631aa85f932e5ee33aae1f7b8a0538e
2 files changed