docs(@angular-devkit/schematics): update Copilot rules

Remove the `mutate` method from State management section

Author: bampakoa

packages/schematics/angular/ai-config/files/__rulesName__.template

@@ -34,7 +34,6 @@
 - Use signals for local component state
 - Use `computed()` for derived state
 - Keep state transformations pure and predictable
-- Do NOT use `mutate` on signals, use `update` or `set` instead
 
 ## Templates