Convert target_submit_cmd() to do its own calls and then remove
target_submit_cmd_map_sgls() since no one uses it.

Link: https://lore.kernel.org/r/20210227170006.5077-15-michael.christie@oracle.comTested-by: Laurence Oberman <loberman@redhat.com>
Reviewed-by: Bodo Stroesser <bostroesser@gmail.com>
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>