cleanup email setup for bridges@tpo in polyanthum

The new version of bridgedb is deployed. It's ready to remove esmtp/dkimverify/procmail and install opendkim for bridges@torproject.org.

Related to #40622 (closed).

Update from anarcat: for background, the old setup used procmail to pipe mails into dkimverify and then esmtp to deliver the mail to the old bridgedb service. in #40581 (closed) we implemented a simpler way of doing delivery (removing procmail/esmtp) and in #40622 (closed) we implemented postfix-level dkim checks. now, we want to remove that legacy stack. the work here involves:

• add a transport rule (or whatever was done in #40581 (closed)) to route mails directly to the new bridgedb deployment (probably using the same configuration used by esmtp, but using postfix)
• test deployment to see if things still work, probably in coordination with @meskio
• if all that works, remove old crap, which means;
• remove the virtual_alias_maps from main.cf
• ... and the actual files
• remove the mailbox_command delegated to #40639 (closed)
• uninstall procmail, we can't, it's a userdir-ldap dependency, see #40639 (closed)
• esmtp,
• dkimverify
• remove old procmail configs, logs, possible mailboxes