Remove duplicated source listing of gtest_prod_util.h

This should have been done in
https://webrtc-codereview.appspot.com/39579004

BUG=
R=andresp@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/46779004

Cr-Commit-Position: refs/heads/master@{#8908}
2 files changed