searcher: remove batch API v2
Created by: camdencheek
Now that the stream API is enabled by default, and appears to be stable, we can remove the unused batch API.
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.
Reopened from #24021 Closes #23981 (closed)