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Remove most of the old assistant code. This is the first pass, will do
a second pass to catch the remaining.

Bug: b:120553209
Test: locally build and run
Change-Id: I1907ea797ef42a5e1f43ddc6200986e4184661e5
Reviewed-on: https://chromium-review.googlesource.com/c/1388441
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621425}