btrfs: remove max_active var from open_ctree

Introduced by 5cdc7ad3 ("btrfs: Replace fs_info->workers with
btrfs_workqueue.") but obsoleted by 2a458198 ("btrfs: factor
btrfs_init_workqueues() out of open_ctree()").

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>