I'm sure this commit was largely motivated by business reasons, but it seems pretty reasonable from a forward-looking technical standpoint as well.

Google search on Android has gone far beyond an endpoint where you give it a string and it gives you a list of URLs; it gives smart autocomplete, you can search by voice (which is smarter than speech recognition + text search), it provides custom result widgets for various situations, etc. As the feature set expands, it becomes less and less reasonable to have a single client that works against an abstraction implemented by many competing backend services, so the idea of a "search engine picker" makes less and less sense. It's saner to just say "this is the Google search client, and other search services can write their own clients". For example, Bing has its own Android app with its own design and features not in Google search. (That's not to say it's fair; Bing needs to be a separate app, whereas Google search is a first-class part of Android.)