Revert of Linux UI: Switch to the Gtk3 theme (patchset #2 id:160001 of https://codereview.chromium.org/2670623002/ )

Reason for revert:
Broke MSAN bots and ClusterFuzz bots.

BUG=698239,698252

Original issue's description:
> Linux UI: Switch to the Gtk3 theme
>
> Sets "use_gtk3 = true" by default.
>
> BUG=79722
> TBR=dpranke@chromium.org
>
> > Review-Url: https://codereview.chromium.org/2670623002
> > Cr-Commit-Position: refs/heads/master@{#452118}
> > Committed: https://chromium.googlesource.com/chromium/src/+/872a494bba52e597388aec738d9d681183a3d47b
>
> Review-Url: https://codereview.chromium.org/2670623002
> Cr-Commit-Position: refs/heads/master@{#454491}
> Committed: https://chromium.googlesource.com/chromium/src/+/89be63a39b5014096115d3ec60d13fde39e73283

TBR=erg@chromium.org,mmoss@chromium.org,thomasanderson@google.com
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=79722

Review-Url: https://codereview.chromium.org/2722313008
Cr-Commit-Position: refs/heads/master@{#454606}
4 files changed