Remove dead test cases in webrtc benchmark

Bug:468732

These test cases have been failing completely for a long time now. Remove them
to clean up code debt.

Change-Id: I1743df497809a21ecda23b256691ccde9ecb953a
Reviewed-on: https://chromium-review.googlesource.com/833889
Reviewed-by: Patrik Höglund <phoglund@chromium.org>
Commit-Queue: Ned Nguyen <nednguyen@google.com>
Cr-Commit-Position: refs/heads/master@{#525026}