Update mention bot config

Removes some of the values which are already set to their default values
on mention bot.

.mention-bot

@@ -1,11 +1,7 @@
 {
-  "maxReviewers": 3, // 3 is enough
-  "numFilesToCheck": 5,
+  "numFilesToCheck": 10,
   "message": "@pullRequester, thanks! @reviewers, please review this.",
-  "findPotentialReviewers": true,
-  "fileBlacklist": ["*.md"], // ignore documentation
-  "userBlacklist": ["ngtuna", "janetkuo", "sebgoa"], // explicitly ignore pinging alumni
-  "assignToReviewer": false,
-  "createReviewRequest": true,
-  "createComment": true,
+  "fileBlacklist": ["*.md"],
+  "userBlacklist": ["ngtuna", "janetkuo", "sebgoa"],
+  "actions": ["opened", "labeled"]
 }