Revert of DevTools: launch What's New from experiments (patchset #1 id:1 of https://codereview.chromium.org/2749423003/ )

Reason for revert:
It needs more UX work.

Original issue's description:
> DevTools: launch What's New from experiments
>
> This is intended to gather feedback in Canary and then
> we'll add another set of release notes before the next branch point.
>
> BUG=682879
>
> Review-Url: https://codereview.chromium.org/2749423003
> Cr-Commit-Position: refs/heads/master@{#457598}
> Committed: https://chromium.googlesource.com/chromium/src/+/9f061aa07fd7c89d8f1b0c3ddedc158c60939220

TBR=chenwilliam@chromium.org
NOTRY=true
BUG=682879

Review-Url: https://codereview.chromium.org/2759223002
Cr-Commit-Position: refs/heads/master@{#458182}
3 files changed