Fix caching of VISIT_CONFIG_SITE

[markcmiller86]

Description

This is to fix caching of VISIT_CONFIG_SITE command-line option so that re-run attempts will remember the setting.

I also removed all modification comments in CMakeLists.txt prior to 2020.

Type of change

• [x] Bug fix~~
• [ ] New feature
• [ ] Documentation update
• [ ] Other

How Has This Been Tested?

Reminders:

• Please follow the style guidelines of this project.
• Please perform a self-review of your code before submitting a PR and asking others to review it.
• Please assign reviewers (see VisIt's PR procedures for more information).

Checklist:

• [ ] I have commented my code where applicable.
• [x] I have updated the release notes.~~
• [ ] I have made corresponding changes to the documentation.
• [ ] I have added debugging support to my changes.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [x] I have confirmed new and existing unit tests pass locally with my changes.~~
• [ ] I have added new baselines for any new tests to the repo.
• [x] I have NOT made any changes to protocol or public interfaces in an RC branch.~~

[markcmiller86]

@cyrush I believe this should fix your issues with VISIT_CONFIG_SITE