Custom Tabs: remove WPR support from customtabs_benchmark script.

This script used the old python WPR wrapper, which has disappared in
favor of a Go-based server which may or may not be publically
supported (see crbug.com/795229). For the time being, it seems expedient
to just tear out WPR, and hence traffic-shaping, support in the script.

Change-Id: Ia001929a110b580fb35ae0cc2b8f8d7c80154caf
Reviewed-on: https://chromium-review.googlesource.com/829034


Reviewed-by: Egor Pasko <pasko@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524404}