Remove the deprecated log_syntax_violation function for 2.0

[est31]

I've also removed the OldFn variant as that function was the only place that variant was constructed.
Does that make sense or should I replace the entire enum with Option's everywhere like it was before #433?

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This change is 

[dekellum]

If you replaced the enum with an Option, you'd still need to wrap in a new-type (generic over the lifetime) to implement the same specific methods for the type, right? Looks good to me, the way you have it.

[SimonSapin]

Merged in the 2.0 branch, with an Option: 48f3bed.

Thanks!

[dekellum]

…or move those methods to Parser and don't bother using them from Input. That works too, I see, and is also more compact.

[est31]

Merge commit was: 90e65b2