s390/zcrypt: Cleanup leftover module code.

The AP bus code is not buildable as kernel module any more.
Commit 5fe38260d083 ("s390/zcrypt: make ap_bus explicitly
non-modular") leaves one now unused function which gets
removed with this patch.

Signed-off-by: Harald Freudenberger <freude@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>