notifications: Remove the stray notification trigger strings.

This commit removes the stray strings used to refer to
various types of notification triggers.

We use the attributes of the 'NotificationTriggers' class instead.

zerver/lib/notification_data.py

@@ -323,7 +323,10 @@ def get_mentioned_user_group_name(
     messages instead.
     """
     for message in messages:
-        if message["mentioned_user_group_id"] is None and message["trigger"] == "mentioned":
+        if (
+            message["mentioned_user_group_id"] is None
+            and message["trigger"] == NotificationTriggers.MENTION
+        ):
             # The user has also been personally mentioned, so that gets prioritized.
             return None