assistant: remove enable hotword libassistant config

Bug: b:123939389
Test: locally build and run
Change-Id: I6e89f7c622d71ab7862e2c3c25d56778b37eac2c
Reviewed-on: https://chromium-review.googlesource.com/c/1455475
Commit-Queue: Xiaohui Chen <[email protected]>
Reviewed-by: Tao Wu <[email protected]>
Cr-Commit-Position: refs/heads/master@{#629728}
9 files changed