Remove unified-consent flag from flag-metadata.json

The corresponding feature flag was already removed from the application
a while back. This CL removes it from flag-metadata.json as well.

Bug: NONE
Change-Id: I6929e19c4c61445b03458b80c3e44ff2e18da068
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2010772
Auto-Submit: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733398}

chrome/browser/flag-metadata.json

@@ -3560,11 +3560,6 @@
     // Chrome OS.
     "expiry_milestone": -1
   },
-  {
-    "name": "unified-consent",
-    "owners": [ "msarda", "tangltom" ],
-    "expiry_milestone": 79
-  },
   {
     "name": "unsafely-treat-insecure-origin-as-secure",
     "owners": [ "security-dev" ],