s390: remove do_signal() prototype and do_notify_resume() function

Both are no longer used since the conversion to generic
entry, therefore remove them.

Fixes: 56e62a73

 ("s390: convert to generic entry")
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>