Updated GPU AppContainer flag expiry to M82.

This CL updates the expiry milestone for the GPU AppContainer flag to
M82. While this file is being modified remove the entry for the old
renderer AppContainer flag which has already been removed from the
codebase.

Change-Id: I75ddee725bc86a1ed0a6063b77a9c2e4c56bf4eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1758342Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Will Harris <wfh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688590}

chrome/browser/flag-metadata.json

@@ -808,11 +808,6 @@
     "owners": [ "knollr", "peter" ],
     "expiry_milestone": 74
   },
-  {
-    "name": "enable-appcontainer",
-    "owners": [ "forshaw", "wfh" ],
-    "expiry_milestone": 76
-  },
   {
     "name": "enable-arc-cups-api",
     "owners": [ "luum", "skau" ],
@@ -1207,7 +1202,7 @@
   {
     "name": "enable-gpu-appcontainer",
     "owners": [ "forshaw", "wfh" ],
-    "expiry_milestone": 78
+    "expiry_milestone": 82
   },
   {
     "name": "enable-gpu-rasterization",