s390/base: remove unused s390_base_mcck_handler

s390_base_mcck_handler was used during system reset if diag308 set was
not available. But after commit d485235b ("s390: assume diag308 set
always works") is a dead code and could be removed.

Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>