Delete obsolete flag metadata entry.

This flag was removed some time ago, but its metadata remained.

Change-Id: Ibf23e733b244b96e7b66cc2cbbe76ae0f5d6cf19
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1838956
Commit-Queue: Adam Langley <agl@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702910}

chrome/browser/flag-metadata.json

@@ -1905,11 +1905,6 @@
     "owners": [ "webauthn-team@google.com" ],
     "expiry_milestone": 84
   },
-  {
-    "name": "enable-web-authentication-pin-support",
-    "owners": [ "webauthn-team@google.com" ],
-    "expiry_milestone": 77
-  },
   {
     "name": "enable-web-authentication-testing-api",
     "owners": [ "webauthn-team@google.com" ],