Revert "assistant: add audio ducking for internal media"

This reverts commit f7860793f8be2946d367b8ece0d7e08ad3d51f78.

Reason for revert: suspect causing compile failure on linux-chromeos-google-rel.

Sample build: https://ci.chromium.org/p/chrome/builders/ci/linux-chromeos-google-rel/8776

Original change's description:
> assistant: add audio ducking for internal media
> 
> Bug: b/135064564
> Test: locally build and run with multiple media playing
> Change-Id: I3814cfc3bc619077ca6781493fd369c27d50eb4e
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1899184
> Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
> Reviewed-by: Tao Wu <wutao@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#713200}

TBR=xiaohuic@chromium.org,wutao@chromium.org

Change-Id: I6595ecc2d3ea93a762996386f3af9bf297daed70
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: b/135064564
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1902180
Reviewed-by: Takashi Sakamoto <tasak@google.com>
Commit-Queue: Takashi Sakamoto <tasak@google.com>
Cr-Commit-Position: refs/heads/master@{#713292}
6 files changed