Remove both rietveld config and review_lgtm setting from cq config

tools/infra/config/cq.cfg

 # See http://luci-config.appspot.com/schemas/projects/refs:cq.cfg for the
 # documentation of this file format.
 version: 1
 cq_name: "dart"
 git_repo_url: "https://dart.googlesource.com/sdk.git"
-rietveld {
-  url: "https://codereview.chromium.org"
-}
 gerrit { }
 verifiers {
-  reviewer_lgtm {
-     committer_list: "dart-team"
-     dry_run_access_list: "dart-team"
-  }
   gerrit_cq_ability {
      committer_list: "dart-team"
      dry_run_access_list: "dart-team"

[tandrii(chromium), 2017/06/30 19:07:42]

nit: not for this CL, but it's be great if you used groups named according to
our typical naming scheme:

project-dart-committers
project-dart-tryjob-access

each of these can still include dart-team.

   }
   try_job {
     buckets {
       name: "tryserver.client.dart"
       builders { name: "vm-win-debug-x64-try"}
       builders { name: "vm-win-release-x64-try"}
       builders { name: "vm-linux-debug-x64-try"}
       builders { name: "vm-linux-release-x64-try"}
       builders { name: "vm-mac-debug-x64-try"}
       builders { name: "vm-mac-release-x64-try"}
     }
   }
 }