btrfs: Remove redundant btrfs_release_path from btrfs_unlink_subvol

Although it is safe to call this on already released paths with no locks
held or extent buffers, removing the redundant btrfs_release_path is
reasonable.

Signed-off-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>