s390/atomic: Remove __atomic_cmpxchg() variants

With users converted to the standard arch_cmpxchg() variants, remove
the now unused __atomic_cmpxchg() and __atomic_cmpxchg_bool() variants.

Reviewed-by: Juergen Christ <jchrist@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>