Removing old Database2 converters

[john-science]

What is the change?

Removing references to versions of the database prior to Database3:

• armi.bookkeeping.db.convertDatabase()
• armi.cli.database.ConvertDB

Why is the change being made?

This version of the ARMI Database was introduced more than 4.5 years ago, and these tools were introduced to help people make the transition. The transition is over.

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Checklist

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

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

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

[john-science]

@drewj-usnctech I wanted to check that this change won't affect you. I can't imagine it will, but just to be sure.

[opotowsky]

Love a good delete-only PR :-)