Remove undocumented option --skip-requirements-regex

[chrahunt]

What's the problem this feature will solve?

Extra undocumented command-line argument/configuration parameter potentially complicating debugging and code refactoring.

Describe the solution you'd like

Deprecation of --skip-requirements-regex and then removal in 20.1.

Alternative Solutions

• Document this option and fully support it - In general we're moving away from these kind of workarounds - if this were suggested today I do not think it would be given much consideration
• Remove the option with no deprecation period - not preferred after some discussion in Remove undocumented --skip-requirements-regex #7188

Additional context

• Remove undocumented --skip-requirements-regex #7188
• pip is growing a lot of configuration settings/settings #6221
• No mentions of this option online except Handling of local Python dependencies dependabot/feedback#252 or copies of the pip source code