s390/smp: Remove conditional emergency signal order code usage

pcpu_ec_call() uses either the external call or emergency signal order
code to signal (aka send an IPI) to a remote CPU. If the remote CPU is
not running the emergency signal order is used.

Measurements show that always using the external order code is at least
as good, and sometimes even better, than the existing code.

Therefore remove emergency signal order code usage from pcpu_ec_call().

Suggested-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>