Remove user-activation-v2 from flag-metadata.json.

The flag was removed through https://crrev.com/c/1873384.

Bug: 959850
Change-Id: I49916760f106bb5e07f468eb9bb04e547b52d231
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1918101Reviewed-by: Navid Zolghadr <nzolghadr@chromium.org>
Commit-Queue: Mustaq Ahmed <mustaq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715474}

chrome/browser/flag-metadata.json

@@ -3593,11 +3593,6 @@
     "owners": [ "toyoshim" ],
     "expiry_milestone": 83
   },
-  {
-    "name": "user-activation-v2",
-    "owners": [ "mustaq", "input-dev" ],
-    "expiry_milestone": 76
-  },
   {
     "name": "username-first-flow",
     "owners": [ "dvadym" ],