CMake: Don't clobber un-namespaced options

[ZedThree]

Fixes #538

Non-SUNDIALS namespaced variables may be used by other projects so don't unset them

• Please target the develop branch not main.
• Review our Contributing Guide, and ensure that you sign your last commit (at minimum) as per the guide.
• Provide a concise description of what your pull request does, and why it is needed/benefical.
• Add a note about your change to the CHANGELOG.md and docs/shared/RecentChanges.rst files. Notice that the former is a markdown file and the latter is reStructuredText, so the formatting is slightly different.
• After your PR is opened, ensure that all of the tests are passing (a SUNDIALS developer will have to allow the testing to run).

[balos1]

Looks good to me. This will make the CMake GUI display options that are irrelevant unless the right ENABLE_* variable is set, but we will have to find a different way to address that.