Now that we have migrated to version level task generation, we should go cleanup the old build-variant level task generation code. This way we don't have to maintain it or worry about anyone making changes to it thinking it is being used.

It looks like some other code paths have coupled themselves to the code (burn_in_tests, selected tests), so this might require some refactoring to completely get out all the unused code.

As a database contributor,
I want unused build_variant generation code to be removed,
So that I don't have to maintain it.

AC:

• evergreen_build_variant_gen script is deleted.
• Other unused code related to task generation is removed.
• Extract out portions of the code that is needed for other commands