bcachefs: delete_dead_snapshots() doesn't need to go RW

We've been moving away from going RW lazily; if we want to go RW we do
that in set_may_go_rw(), and if we didn't go RW we don't need to delete
dead snapshots.

Reported-by:  <syzbot+4366624c0b5aac4906cf@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>