removed workflow step for publishing python 3.8 images

[cicirello]

Summary

Removed workflow step for publishing python 3.8 images. Python 3.8 is end-of-life, and no longer maintained. New policy for building images for older Python versions is to do so only for versions that are still supported. See Status of Python Versions.

Types of changes

• I have read the CONTRIBUTING document.
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvements to existing code, such as refactoring or optimizations (non-breaking)
• Breaking change (fix or feature that would cause existing functionality to change)
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.