🚀 Feature: Remove the long-deprecated --opts option

[JoshuaKGoldberg]

Feature Request Checklist

• I have read and agree to Mocha's Code of Conduct and Contributing Guidelines
• I have searched for related issues and issues with the faq label, but none matched my issue.
• I want to provide a PR to resolve this

Overview

As of #4175, mocha.opts is deprecated and non-functional. I've yet to see anybody use it in real projects. But the code still exists:

mocha/lib/cli/run.js

Lines 341 to 346 in e3da641

	if (argv.opts) {
	throw createUnsupportedError(
	`--opts: configuring Mocha via 'mocha.opts' is DEPRECATED and no longer supported.
	Please use a configuration file instead.`
	);
	}

This is dead weight on the project. IMO there's no real need to keep it around given how it's been deprecated with a loud warning & migration path (using a config instead) for 5 years.

Suggested Solution

I propose we remove it in a future major version, such as Mocha 12.

Alternatives

We could leave it in, but all it does now is throw createUnsupportedError.

Additional Info

Similar sibling issue: #5242

[JoshuaKGoldberg]

Marking as accepting PRs. Note that we won't be able to merge it until we start merging PRs for Mocha v12. Which will be an unknown, not-very-soon time from now.