Revert "Enable chrome.metricsPrivate for the untrusted context of MediaApp so we can use UMA."

This reverts commit bbbb72b3.

Reason for revert: MediaAppUIBrowserTest failing on linux-chromeos-chrome
Fixed: 1044503

Original change's description:
> Enable chrome.metricsPrivate for the untrusted context of MediaApp so we can use UMA.
> 
> Doc: http://go/backlight-ea-uma-metrics (slightly outdated)
> 
> Bug: 996088, b/128567301
> Change-Id: Ia272a35815ac92de954a6e5c43e1c2d4ea502e0d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2000440
> Reviewed-by: Trent Apted <tapted@chromium.org>
> Reviewed-by: Ben Wells <benwells@chromium.org>
> Commit-Queue: David Lei <dlei@google.com>
> Cr-Commit-Position: refs/heads/master@{#733884}

TBR=tapted@chromium.org,benwells@chromium.org,dlei@google.com

Change-Id: I7e95b2c95bb5b1abd2f19416e585ae6f7f6f404b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 996088, b/128567301
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2014354Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Commit-Queue: Yoichi Osato <yoichio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733904}

extensions/common/api/_api_features.json

@@ -309,7 +309,6 @@
     "matches": [
       "chrome://bookmarks/*",
       "chrome://extensions/*",
-      "chrome://media-app-guest/*",
       "chrome://os-settings/*",
       "chrome://settings/*",
       "chrome://sync-confirmation/*",