Remove clank sharing hub flag

Remove remaining references to the ChromeSharingHub Feature.
It was only being used to parse the finch param, and this
can be done by using the feature name directly.

Bug: 1234422
Change-Id: I0b227c452087d58c28d2f096399c9d4a2fa77d76
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3064858


Reviewed-by: Sophey <sophey@chromium.org>
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#908691}