btrfs: remove deprecated mount option subvolrootid

The option subvolrootid used to be a workaround for mounting subvolumes
and ineffective since 5e2a4b25 ("btrfs: deprecate subvolrootid mount
option"). We have subvol= that works and we don't need to keep the
cruft, let's remove it.

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