More cleanup of GYP code in ui/ozone

GYP files for Ozone have been removed in
https://codereview.chromium.org/2306743002

We remove some comments from the GN files referring to these GYP files.
We also remove the file ui/ozone/empty.cc, which is not used anymore.

R=rjkroege@chromium.org
BUG=none

Review-Url: https://codereview.chromium.org/2312763002
Cr-Commit-Position: refs/heads/master@{#416871}
3 files changed