Bug 1625099 - no need to wait for `metadatachange`. r=bryce

When controller gets deactivated, we would have had reset the display metadata already, so we can remove the promise for `metadatachange`.

The reason of causing failure is because `metadatachange` sometime would race with `deactivated`. If `metadatachange` gets dispatched after `deactivated`, we would miss that event.

The actual fix for event race will be handled in bug 1688507, this patch is simply fixing the failure.

Differential Revision: https://phabricator.services.mozilla.com/D102871

dom/media/mediacontrol/tests/browser/browser_media_control_metadata.js

@@ -341,7 +341,6 @@ add_task(async function testMetadataAfterTabNavigation() {
   await Promise.all([
     new Promise(r => (tab.controller.ondeactivated = r)),
     BrowserTestUtils.loadURI(tab.linkedBrowser, "about:blank"),
-    waitUntilDisplayedMetadataChanged(),
   ]);
 
   info(`current media metadata should be reset`);