Cleanup unused code in AnnotatedString

[vmartinv]

I think these functions are not used anymore after the latest refactor. The reason I want to remove it is because I'll be adding a bunch of tests to improve coverage, and this code is not really reachable.

I also correct an error in a comment's example (Range.overlaps uses exclusive ending ranges).

[sylvainhalle]

I thank you but if you don't mind I'd leave this specific PR on hold for now. In fact I'd eventually like to move AnnotatedString back into the Petit Poucet library instead of TeXtidote, and to do so it is important that the class implements ExplanationQueryable.

[vmartinv]

Got it, then it won't hurt to keep it. Maybe I will add some tests later.