btrfs: remove waitqueue_active check from btrfs_rm_dev_replace_unblocked

Normally the waitqueue_active would need a barrier, but this is not
necessary here because it's not a performance sensitive context and we
can call wake_up directly.

Suggested-by: Chris Mason <clm@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>