btrfs: remove unused btrfs_cond_migrate_bytes

The last user of this was removed in 7f9fe614 ("btrfs: improve
global reserve stealing logic"), drop this code as it's no longer called
by anybody.

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