chore(spanner): centralize mypy config for spanner

[chalmerlowe]

• Relies on a centralized mypy.ini file at the root of the repository to consolidate all shared and package-specific mypy exceptions.
• Deletes the individual mypy.ini files from handwritten libraries, establishing the root configuration as the source of truth.
• Refactored their noxfile.py files to define MYPY_CONFIG_FILE dynamically and standardized their session.run("mypy", ...) invocations.

Related to: PR #17409 and PR #17640

[gemini-code-assist]

Code Review

This pull request centralizes the mypy configuration by removing the package-specific configuration file in packages/google-cloud-spanner and updating its noxfile.py to reference the root mypy.ini. The root configuration is also updated with various package-specific overrides. However, the override to ignore missing imports for google.iam.* from the deleted configuration was not migrated to the root mypy.ini, which will cause mypy checks to fail for the spanner package.