Commit a185e2e3 removed the Rietveld code review
implementation, albeit hasn't expunged all references to Rietveld.  This leads
to an assertion error when running |git cl format| on a non-Gerrit change.
There appears to have been a recent attempt to articulate this error better in
a185e2e3, albeit not fix the underlying issue.
This change addresses the immediate issue by defaulting the code review
implementation to Gerrit rather than Reitveld.

R=joenotcharles@google.com, tandrii@chromium.org

Bug: 914150
Change-Id: I64d33e5a172cc43339ec417f7f0a7820e0337772
Reviewed-on: https://chromium-review.googlesource.com/c/1372928Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Bryce Thomas <bryct@amazon.com>