Remove backend Admin::Search controller

[jhawthorn]

Split from #240

This controller has two "lightweight" versions of existing api index actions. We should switch to using the API, but it's currently significantly slower.

We'll have to either optimize the endpoint, or allow specifying which fields we want (ex. ?fields=name,id or ?include=)

[bbuchalter]

Now that #240 is merged, is there any reason we can't remove Admin::Search?

[bbuchalter]

Firing up a dummy app and running rake routes shows admin_search_users and admin_search_products are still out there, but not in use anywhere. I suppose we'd need to deprecate those end points? If there is a good example of how to do that, I'd be happy to open a PR. Or perhaps we just remove them?

[cbrunsdon]

Marked it down on my personal hit list, but closing this issue due to staleness.