Move mention-bot notifications to code owners

Signed-off-by: Joas Schilling <coding@schilljs.com>

.github/CODEOWNERS

+*/Activity/*                        @nickvergessen
+*/Notifications/*                   @nickvergessen

.mention-bot

-{
-  "maxReviewers": 3,
-  "numFilesToCheck": 5,
-  "alwaysNotifyForPaths": [
-    {
-      "name": "nickvergessen",
-      "files": [
-        "lib/private/Activity/**",
-        "lib/private/Notification/**",
-        "lib/public/Activity/**",
-        "lib/public/Notification/**"
-      ]
-    },
-    {
-      "name": "Xenopathic",
-      "files": [
-        "apps/files_external/**"
-      ]
-    }
-  ],
-  "userBlacklist": [
-    "DeepDiver1975",
-    "nextcloud-bot",
-    "owncloud-bot",
-    "PVince81",
-    "scrutinizer-auto-fixer",
-    "th3fallen",
-    "zander",
-    "luckydonald",
-    "jancborchardt"
-  ],
-   "createReviewRequest": true,
-   "createComment": false
-}