uninstall --unused: Don't prompt if no unused refs found

If there was no refs found to delete, we don't want to print the "ok?" prompt.
Instead we just say that nothing was found.

Closes: #1776
Approved by: alexlarsson
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f