TRAP Caching: Unset missing cache rather than setting to undefined

[edoardopirovano]

In other parts of the code, we expect the values in the map to not be undefined, so we are getting warnings like https://github.com/github/vscode-codeql/actions/runs/2911692423 if we fail to find a cache in the Actions cache because we are setting a value of undefined here. Let's just remove the language from the map entirely if we don't have a cache for it.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[henrymercer]

LGTM. When we set the property undefined, it still appears in Object.values(...). Deleting it removes it from Object.values(...), so we don't attempt to find its size.