WebUI: Remove new-material style on cr-tabs

The side panel is no longer using this new style of tabs and was the
only UI using it. Removing new-material entirely since it was originally
buggy anyway.

This is essentially a revert of https://chromium-review.googlesource.com/c/chromium/src/+/2879522.

Change-Id: Ic2a55b2153c7029d1ff0084534f00cfadf2f62ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2919183


Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#887359}