ci: remove workflow: lifecycle (#1406)

This change removes //.github/workflows:lifecycle.yml, which was
historically used to automatically label and unlabel issues and pull
requests based on inactivity. After consideration, it has been decided
that this sort of automatic label management introduces unecessary
noise.

The removal of this workflow means that issues and pull requests will no
longer be marked as idle or dormant. These labels can be removed:

- lifecycle/idle
- lifecycle/dormant

Change-Id: I03d8cc6cf11d8fde88029bd64315855d81e3d353