flags: remove save-page-as-mhtml

We'll retain the switch for developer use. See offline-dev:
https://groups.google.com/a/chromium.org/d/msg/offline-dev/qO1B8kkWkoM/kO30ODfRBgAJ

Bug: 120416
Change-Id: Id4d9a5b5c908d2f47b2c667ea5724101e3ed7094
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529254
Auto-Submit: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641987}
4 files changed