Revert "ANDROID: Build null_blk and scsi_debug as kernel modules"

Revert commit 9b476ebe in preparation of making this a
Pixel-specific change. A disadvantage of building null_blk and
scsi_debug as kernel modules for all vendors is that this causes these
kernel modules to be loaded by the init process during boot and there is
no vendor-independent mechanism for preventing kernel modules to be
loaded.

Bug: 323406846
Change-Id: I6e7bcdd9fd06194bbc134ee52251b6ae9a3c1477
Signed-off-by: Bart Van Assche <bvanassche@google.com>