Misc cleanup

[john-science]

What is the change?

1. I removed some random code (AVAILABLE_MODIFICATION_NAMES)
2. I removed an unused armi/physics/safety/settings.py file.
3. I added requirements to our PR template.

Why is the change being made?

• @keckler pointed out both (1) and (2) above.
• (3) above came out of discussion with @albeanth

---

Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify that the new/changed code works.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes (location doc/release/0.X.rst) are up-to-date with any important changes.
• The documentation is still up-to-date in the doc folder.
• No requirements were altered.
• The dependencies are still up-to-date in pyproject.toml.

[john-science]

I see the test failure. Oddly, it passes for me locally. But I'm looking at it.

Fixed.