add an option to find all //FI comments that are no longer necessary

Since every new version of FixInsight fixes some false positives which we previously had to disabled using a //FI:<warning-code>, these comments now become obsolete and clutter the code. Please either add an option or maybe even always add a message about such unnecessary comments.

(I think an option would be better, sometimes I only want to see new warnings, and don't care about obsolete //FI comments.)