Kconfig: remove comment about scsi_wait_scan module

This module has been removed in commit 6072609d


([SCSI] Remove scsi_wait_scan module), so this module is gone since 3.6.

Signed-off-by: Rolf Eike Beer <eike-kernel@sf-tec.de>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>