[libc++] Remove transitional #error message

It's been more than 4 years now, so anyone that was defining
_LIBCPP_TRIVIAL_PAIR_COPY_CTOR has had ample time to see that
error and fix their code.

GitOrigin-RevId: f1cf6b47e4b659bae3314acbb0d11d533bd4175f
1 file changed