chore: Delete obsolete manual mypy.yml workflow

Since we consolidated the mypy check into the unified continuous-integration.yml pipeline (running differential checks on PRs), the manual strict mypy.yml workflow is redundant and has never been triggered in the history of the repository.

Deleting this dead configuration to keep the Actions sidebar clean and avoid configuration rot.

Co-authored-by: Shangjie Chen <deanchen@google.com>
PiperOrigin-RevId: 936400372

Author: DeanChensj