Drop unnecessary validation during execute_summary_view()

Hi. First of all I want to thank you for the module, it is really rocks! :)

We are using Views Calc for our ERPAL distribution. But recently we met a serious performance issues on a pages, where we use views calc table for a display format. I've noticed that each time when invokes execute_summary_view() method, it invokes views validation. That validation runs for each display of this view, and invokes validation for each handler in each display. Validation for handlers may be very time consuming. For example, entityreference handler checks accesses to the each entity. And if there are a lot of referenced entities - this validation may takes tens and hundreds of seconds. Also please keep in mind that displays of the same view may contain different handlers.

So the problem is that there are possible situation (that we met), when Views Calc invokes heavy validation for a view (on page), but some of the handlers are not even exists on that page! And the more view displays we have - the worse performance we have.

Also I am not sure that Views Calc should do this validation at all, because in general, this is a Views job to take care about proper work of handlers and their data.

My proposal is to simply drop the lines with view validation.

Looking forward for your response!