Cleaning up docstring and unused instance variable

[albeanth]

What is the change?

While doing a review of depletion capabilities I noticed that the docstring for the Assembly class was a bit out of date and that the instance variable pinPeakingFactors was unused. This PR cleans that out.

Why is the change being made?

No need to keep stuff that's not used.

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Checklist

• [x] This PR has only one purpose or idea.
• ~Tests have been added/updated to verify any new/changed code.~

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

• [x] The release notes have been updated if necessary.
• [x] The documentation is still up-to-date in the doc folder.
• [x] The dependencies are still up-to-date in pyproject.toml.