Deleting stale merged branches on 19th February

[Benoit]

As noted in https://groups.google.com/g/metamath/c/TflSBA0jZpM and https://github.com/metamath/set.mm/issues/2476, branches in the repository  metamath/set.mm are now automatically deleted once they get merged.  This change did not deal with the "backlog" of old merged branches.  Therefore, unless you object (in general, or for a specific branch), starting on 19th February (ten days from now), I'll delete the stale, merged branches in metamath/set.mm (listed at https://github.com/metamath/set.mm/branches/stale).  Note that this happens on metamath/set.mm, and not on your own fork.

Benoît