All CQ supported projects already have git mirrors. We don't
want to support svn checkouts anymore.

BUG=503847

Review URL: https://codereview.chromium.org/1213363002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@295882 0039d316-1c4b-4281-b951-d872f2087c98