Fix failing spec on PostgreSQL
Fix failing spec on PostgreSQL

In order to use `distinct` in `Spree::Product#variant_images` association,
the ordering of the joined relations must be removed.

It's possible to leave the other association ordering criteria by using an
explicit `select` statement but then eventually `count` fails (`count(:all)`
succeeds, but still it's suboptimal).

solidusio/solidus_asset_variant_options,solidusio/solidus_asset_variant_options