bcachefs: Kill equiv_seen arg to delete_dead_snapshots_process_key()

bcachefs: Kill equiv_seen arg to delete_dead_snapshots_process_key()

When deleting dead snapshots, we move keys from redundant interior
snapshot nodes to child nodes - unless there's already a key, in which
case the ancestor key is deleted.

Previously, we tracked via equiv_seen whether the child snapshot had a
key, but this was tricky w.r.t. transaction restarts, and not
transactionally safe w.r.t. updates in the child snapshot.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>