bot: deprecate (un)register_url_callback() functions

[dgw]

Description

Tin. They were a stopgap, and they have served their purpose.

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make quality (did not touch anything testable)
• I have tested the functionality of the things this change touches

[dgw]

Bit concerning that a random test (fittingly, in rand.py) failed on py2, but the same CI job passed when restarted the next day… Would love to know how sopel/modules/rand.py::test_example_rand_4 ended up with 9 outputs instead of 10 on an unrelated PR check, but it's not a problem with this patch.