[MAINT]: Remove deprecated `pvlib.atmosphere.first_solar_spectral_correction`

[echedey-ls]

• [x] Closes #2130
• [x] I am familiar with the contributing guidelines
• [x] Tests removed
• ~[ ] 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`).
• ~[ ] 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.

Pending whatsnew entry. I propose waiting for v0.12 since this is a breaking change, thou I doubt we need to be very careful with a function already deprecated for a whole minor version.