btrfs: drop unused parameter map from scrub_simple_mirror()

The parameter map used to be passed to scrub_extent() until
e02ee89b ("btrfs: scrub: switch scrub_simple_mirror() to
scrub_stripe infrastructure"), where the scrub implementation was
completely reworked.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>