s390: remove is_32bit_task() helper

is_32bit_task() used to be helpful when we still had CONFIG_32BIT.
Since that is gone, it is nowadays identical to is_compat_task().
So remove it.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>