Prevent variable pricing from being affected by price parameter

https://github.com/easydigitaldownloads/Easy-Digital-Downloads/issues/1372

I could be wrong, but I can't think of a single instance where someone would want to remove the variable pricing options, yet keep the purchase button there by itself. If the variable pricing options are removed, there's no way to select an option.

Because of this, I think variable pricing should be unaffected by the price parameter. This seems to me like it would solve the issue entirely. Thoughts?

In fact `price` should probably be renamed to something more suitable such as "show price on button", since this is all it seems to be doing.

I've tested these code changes and all works good from my end. I can now disable button pricing, yet variable pricing still remains.