[Media Router] Remove obsolete flags/features.

Removes the following flags/features:
- load-media-router-component-extension
- cast-media-route-provider
- kCastToMeetingFromCastDialog
- kCastFeedbackDialog
- kDialMediaRouteProvider

Bug: 1179785,1028753
Change-Id: I1d422d629eccc7ff612d0b9c915011183d65ca15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3063781


Commit-Queue: mark a. foltz <mfoltz@chromium.org>
Auto-Submit: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: Filip Gorski <fgorski@chromium.org>
Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#908260}