The purpose of this CL is to remove confusion around whether or
not git-cl support the similarity flags when talking to Gerrit.
It does not. However, because Rietveld support has not been
fully ripped out of git-cl, this change does modify some of the
Rietveld-specific code to remove similarity support from everywhere.

Bug: 800902
Change-Id: I8703ed60d6889c7a36400b8a9f8f26de4669020a
Reviewed-on: https://chromium-review.googlesource.com/912389Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>