nptl: Move cnd_signal into libc

The symbol was moved using scripts/move-symbol-to-libc.py.

The __pthread_cond_signal@@GLIBC_PRIVATE symbol is no longer
neded, so remove that as well.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>