bot, coretasks: remove obsolete URL callback mechanism

[dgw]

Description

While we were happily discussing some future ideas on IRC today, I had a sudden flash of pure hatred for the fact that Sopel.call() still existed, and it felt very good to rip it out along with the only (deprecated) feature still using it.

I no longer want to wait until 9.0 dev starts to do this… Stuff like #2325 or #2435 would become much easier to implement without two different "call the thing" pipelines to worry about. In fact, the rate-related decorators already don't have the same effect when used on callables registered via this deprecated method (they don't handle the message added in #2290).

Vote in the poll on Twitter or Fediverse about whether it would be OK to do this in Sopel 8.0 instead of 9.0, as was promised in the upgrade guide and documentation for Sopel 7.x. (I'm hoping for a landslide voting result, so I can assign a milestone to this patch based on it. 🙃)

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

Not that GitHub is The Only Place Sopel Plugins Exist, but I did a lang:Python code search here for register_url_callback and found 71 matching files, all of which were in clones of this repo. Not a one was a third-party plugin.