Remove unused check on product images

Only the @product.variant_images are used in the view.

@product.images is just the images on the master variant, which is
included in @product.variant_images

fixes #5766
fix n+1 for admin product images

include the variant includes

fix inconsistent indentation

use map instead of collect

Don't load @variants on index

They aren't used on index, they're only used in the form.

Pull out eager loads on index from others

We only access images through the variant_images association, which
isn't covered through the variants_including_master

Variants_including_master will speed up the form, and this will speed up
the index.

Cache the result of has_products?

Rather than calling it twice... which will hit the database

fixes #5723