Remove expired enable-chrome-duet* flag metadata.

Both enable-chrome-duet and enable-chrome-duet-labels have been expired
for a while, and have no references anywhere else.

Change-Id: Ic73397dcb831d5472172a31af7384738abdc81bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2415236Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807992}

chrome/browser/flag-metadata.json

@@ -1277,16 +1277,6 @@
     "owners": [ "mattreynolds", "odejesush", "vickymin@google.com" ],
     "expiry_milestone": 90
   },
-  {
-    "name": "enable-chrome-duet",
-    "owners": [ "mdjones" ],
-    "expiry_milestone": 82
-  },
-  {
-    "name": "enable-chrome-duet-labels",
-    "owners": [ "mdjones" ],
-    "expiry_milestone": 79
-  },
   {
     "name": "enable-clipboard-provider-image-suggestions",
     "owners": [ "rkgibson", "gangwu", "chrome-omnibox-team@google.com" ],