scsi: st: remove redundant pointer STbuffer

Pointer STbuffer is being assigned but is never used hence it is redundant
and can be removed.

Cleans up clang warning:
warning: variable 'STbuffer' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Kai Mäkisara <kai.makisara@kolumbus.fi>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>