Issue #1697 Remove the function and all its reference poa_horizontal_ratio.

[k10blogger]

This pull requests removes the poa_horizontal_ration with all its reference in the documentation and examples.

• [x] Closes #1697
• [x] I am familiar with the contributing guidelines
• [x] Tests added
• [x] Updates entries in docs/sphinx/source/reference for API changes.
• [ ] Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• [x] New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• [x] Pull request is nearly complete and ready for detailed review.
• [ ] Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

[kandersolar]

Does this function deserve a deprecation period before removal? I lean towards no. I doubt it gets much use at all, and anyone who is using it can easily calculate the quantity themselves. But I do think we should at least remove it in a .0 release, so I've tagged this PR for 0.11.0.

Thanks @k10blogger for the PR!

[cwhanse]

Concur with no deprecation and v0.11.0

[AdamRJensen]

@k10blogger this looks good. Could you add a whatsnew entry in v0.11.0?

[cwhanse]

@pvlib/pvlib-maintainer whatsnew used to have "API changes". We dropped that when we switched to "Breaking changes" and "Deprecations" and now I'm not sure where we list a function that's been dropped.

Should we add a section to whatsnew titled "Removals"?

Edit: or list the removal under "Deprecations"?

[kandersolar]

I've understood "Deprecations" to be new deprecations, i.e. usage that now emits a deprecation warning but didn't before, and "Breaking changes" to be functionality that was removed entirely. I'd put the entry for this PR under "Breaking changes".