Deprecate stuff in `Yoast_Input_Validation`

[leonidasmi]

It looks like Yoast_Input_Validation::set_error_descriptions() is no longer useful, ever since we moved to our new settings. Let's see if we can deprecate it and remove it from the textinput(), number() and textinput_extra_content() of the Yoast_Form class.

(Note: please verify that we can indeed do the above without losing any in any edge case)

In fact, it looks like the Yoast_Input_Validation class in general might have other stuff that are no longer useful, so lets have a critical investigation about the whole class too.