MM-9739: deprecate the now unused Channels.ExtraUpdateAt (#8762)

i18n/en.json

@@ -5926,10 +5926,6 @@
     "id": "store.sql_channel.delete.channel.app_error",
     "translation": "We couldn't delete the channel"
   },
-  {
-    "id": "store.sql_channel.extra_updated.app_error",
-    "translation": "Problem updating members last updated time"
-  },
   {
     "id": "store.sql_channel.get.existing.app_error",
     "translation": "We couldn't find the existing channel"