Infra CQ config: remove deprecated target_ref.

BUG=644915
R=sergiyb@chromium.org,machenbach@chromium.org
NOTRY=True

Review-Url: https://codereview.chromium.org/2641523003
Cr-Commit-Position: refs/heads/master@{#42428}

infra/config/cq.cfg

@@ -6,7 +6,6 @@ cq_name: "v8"
 cq_status_url: "https://chromium-cq-status.appspot.com"
 commit_burst_delay: 60
 max_commit_burst: 1
-target_ref: "refs/pending/heads/master"
 
 rietveld {
   url: "https://codereview.chromium.org"