url: remove built-in url plugin

[SnoopJ]

Description

This changeset removes the url built-in plugin as part of #1291, in favor of the newly-extracted sopel-url.

I took the liberty of doing the removal since @Exirel is busy of late

<+xrl> Before I leave for the night, feel free to make the PR that removes url.py from builtins!
<+xrl> Or I'll do it later this month. Or next month. Busy schedule ahead of me!

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 lint and make test)
• I have tested the functionality of the things this change touches