assistant: cleanup dead code

Bug: None
Test: locally build and run
Change-Id: Ifdb6e31b926da18c6da47e7a2a5540f58e9fa9a9
Reviewed-on: https://chromium-review.googlesource.com/c/1423620
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625391}
4 files changed