Remove [no-mentions] handler in our triagebot config (#16361)

This PR removes the `[no-mentions]` handler in our triagebot config as
[GitHub is removing notifications for @-mentions in commit messages on
December 8th
2025.](https://github.blog/changelog/2025-11-07-removing-notifications-for-mentions-in-commit-messages/)

cf. rust-lang/triagebot#2225

Author: epage

triagebot.toml

@@ -30,9 +30,6 @@ label = "O-windows"
 # Documentation at: https://forge.rust-lang.org/triagebot/transfer.html
 [transfer]
 
-# Prevents mentions in commits to avoid users being spammed
-[no-mentions]
-
 # Enable `@rustbot note` functionality
 # Documentation at: https://forge.rust-lang.org/triagebot/note.html
 [note]