Remove hook_views_form_validate() and hook_views_form_submit().

Adding these hooks was a big mistake, because they are pointless.
A Views Form handler can have views_form_validate() and views_form_submit() methods that handle everything.
It can also add additional validation and submit callbacks either through the views_form() method or through a form alter (which is what VBO does).
No point in having a third way of doing it.
Plus, these hooks fire for all views forms, so you always need to check if it's firing on your form and whether you need to actually do anything (which is not the case with other two approaches), which is bad DX.

We never actually recommended that these hooks be used, just documented that they exist.
VBO hasn't used them in a long time, and I've posted a patch to make Draggableviews stop using it: #1477132: Stop using hook_views_form_submit(), move code to a proper submit callback.
Commerce Add To Cart Extras is also not using it.

So, an option could be to just mark those hooks as deprecated, but I'm much more in favor of just removing them.
If there's an obscure module using them, that would be a signal to actually fix their code (and I'd be glad to do it for them).

So let's evaluate the attached patch.