Removed unused field from CQ config

R=phajdan.jr@chromium.org
CC=akuegel@chromium.org
BUG=472612

Review URL: https://codereview.chromium.org/1063423003

Cr-Commit-Position: refs/heads/master@{#324431}

testing/commit_queue/config.json

@@ -22,7 +22,6 @@
     "skip_throttle_users": [
         "commit-bot@chromium.org"
     ],
-    "tryserver_url": "http://build.chromium.org/p/tryserver.chromium/",
     "use_buildbucket_for_tryjobs": true,
     "verifiers_no_patch": {
         "experimental_try_job_verifier": {