Replace dialog with info message for headless operation

[Gemba]

As mentioned here: https://github.com/RetroPie/RetroPie-Setup/pull/3875#issuecomment-1989100380

[cmitu]

I have 2 notes on this change:

• the message can be simplified. Just inform the user that the cache has not been erased and they can erase it by just removing the cache folder or running the skyscraper's purge function.
• don't rename the _purge function, it's used internally elsewhere and also local (to the scriptmodule) functions should keep the _ prefix.

[Gemba]

Thanks for the hints. Trimmed the PR accordingly.

[joolswills]

Thank you.