Remove the Safety scanner from the pre-commit hooks and GitHub action

[pronovic]

Since February, I have been using the Safety scanner as part of my pre-commit hooks and GitHub actions, to identify vulnerabilities in Python dependencies. Even though Safety is distributed under the liberal MIT license, and the PyPI package page documents that Safety can be used in this manner, the PyUp organization behind Safety now claims that this usage is not allowed. Despite my repeated attempts to clarify what I was doing wrong, PyUp's representative never offered any specifics. I don't think Safety adds enough value to pursue this further, so I'm just removing all usage of Safety. I'll rely on Dependabot instead. For more details, see the bizarre email conversation with PyUp that I have captured in the apologies repository.