Remove unused clear cache workflow

[james-garriss]

🗣 Description

My original intention was to move the code in the clear cache workflow into a function and write Pester tests for it, all as part of epic #1402. Then I realized that this workflow is not being used at all. I have decided to remove this workflow instead, and for the moment I put the code into the repo wiki, in case it's ever needed again.

https://github.com/cisagov/ScubaGear/wiki/Clear-Cache-Workflow

💭 Motivation and context

Closes: #1451

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• PR targets the correct parent branch (e.g., main or release-name) for merge.
• Changes are limited to a single goal - eschew scope creep!
• Changes are sized such that they do not touch excessive number of files.
• Related issues these changes resolve are linked preferably via [closing keywords]
• All relevant type-of-change labels added.
• All relevant project fields are set.
• All automated checks (e.g., linting, static analysis, unit/smoke tests) passed.

✅ Pre-merge checklist

• PR passed smoke test check.

• Feature branch has been rebased against changes from parent branch, as needed

• Resolved all merge conflicts on branch

• Notified merge coordinator that PR is ready for merge via comment mention

• Demonstrate changes to the team for questions and comments.
  (Note: Only required for issues of size Medium or larger)

✅ Post-merge checklist

• Feature branch deleted after merge to clean up repository.
• Verified that all checks pass on parent branch (e.g., main or release-name) after merge.

[schrolla]

Reviewed and found no references to the given workflow. So all for removing it.

[dagarwal-mitre]

Couldn't find any references and checked the wiki to make sure code is there, looks good.