searcher: remove batch API
Created by: camdencheek
Now that the stream API is enabled by default, and appears to be stable, we can remove the unused batch API.
I'm making this PR a little early since I don't plan to merge it until the 3.31 release is cut on Thursday. This way, if any customer issues come up, there is still a path to disable the streaming API for at least a full release. Just getting the review process kicked off ahead of time.
Each commit is pretty small and self-contained, so if you'd like to see the progression, review commit-by-commit. Note that all integration tests are now running against the streaming API.
Closes #23981 (closed)