scsi: sd: Remove unnecessary condition in sd_read_block_limits()

After series of changes around WRITE_SAME and UNMAP setup we ended up
with leftover unnecessary condition. Remove it.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>