Remove enable-system-webapp from flag-metadata.json

Bug: 1110666
Change-Id: I5968ba3db606fb07e15bf8972556a80642e8c8db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2483715Reviewed-by: Jiewei Qian  <qjw@chromium.org>
Commit-Queue: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818358}

chrome/browser/flag-metadata.json

@@ -2148,11 +2148,6 @@
     "owners": [ "horo", "//content/browser/web_package/OWNERS" ],
     "expiry_milestone": 87
   },
-  {
-    "name": "enable-system-webapps",
-    "owners": [ "calamity" ],
-    "expiry_milestone": 78
-  },
   {
     "name": "enable-tab-engagement-reporting",
     "owners": [ "memex-team@google.com" ],