Remove [no-mentions] handler in our triagebot config (#16206)

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

changelog: none

Author: samueltardieu

triagebot.toml

@@ -22,9 +22,6 @@ allow-unauthenticated = [
 [mentions."clippy_lints/src/doc"]
 cc = ["@notriddle"]
 
-# Prevents mentions in commits to avoid users being spammed
-[no-mentions]
-
 # Have rustbot inform users about the *No Merge Policy*
 [no-merges]
 exclude_titles = ["Rustup"] # exclude syncs from rust-lang/rust