Panels: Remove panel support in src/extensions and src/chrome.

Panels were deprecated in M-68 and are no longer supported.

Bug: 788033
Change-Id: Ie62deb2b18fda1869ecf4cd1a12a4bdfa668facf
Reviewed-on: https://chromium-review.googlesource.com/1109028
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569376}
20 files changed