Revert "Add output reference to gen written files."

This reverts commit c497d21798caf871326443ad98bd58f546d7b38e.
AKA https://codereview.chromium.org/1252403005/

TBR=dpranke@chromium.org
BUG=533067
TEST=local re-run of gn gen in a few scenarios

Review URL: https://codereview.chromium.org/1381443002

Cr-Original-Commit-Position: refs/heads/master@{#351319}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: a4a5e48c471822d1c51dd00233bf3a167e00d80e