meetbot: remove in favor of external package

[dgw]

Description

Tin. Why?

<+xrl> I'm reviewing the type thing PR. I think we should remove the meetbot plugin.
<+xrl> Reason: I've never seen anyone using it, it's not well tested, and I'm not even sure it actually works.

Draft for now, because:

• Merging Clean up errors in mypy --check-untyped-defs #2471 (the aforementioned "type thing PR") when it's finished will create a conflict
• I'll want to update the extracted plugin file at https://github.com/sopel-irc/sopel-meetbot with the PR's changes to it, anyway
• We probably shouldn't finalize this removal until a package is published, and I haven't converted the new plugin repo to a package yet (because I'm waiting until the first two points are taken care of)

This change leaves the references to meetbot in documentation for how to use the plugin-related settings in the bot's core config section, as they're just examples. No real reason to change them.

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

Notes

As usual I will add a review request to this PR when it's actually ready. I just realized tonight that I'm likely to forget the branch exists if I don't tie it to an open pull request. 😅

[dgw]

If anyone wants to test-run the package, it's in process at sopel-irc/sopel-meetbot#1

[SnoopJ]

The perfect PR doesn't exi-

Easy to approve this one, all red lines!

Might be worth making this the first thing that gets put into NEWS for 8.0 in this changeset, but not having it isn't a big deal for the actual changes.

[dgw]

Might be worth making this the first thing that gets put into NEWS for 8.0

Reminded me that this should get the Breaking Change label, which is exactly what tells me things to mention first in the news, upgrade guide, etc. Thanks.