s390/sclp_sdias: remove unused sclp_sdias_exit

sclp_sdias cannot be built as a module, CRASH_DUMP option is a bool not a
tristate. zcore_exit() has already been removed with commit cbe62fac
("s390: char: make zcore explicitly non-modular"). Remove orphaned
sclp_sdias_exit for consistency as well.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>