black-primer: stop running it (#2809)

At the moment, it's just a source of spurious CI failures and busywork
updating the configuration file.

Unlike diff-shades, it is run across many different platforms and
Python versions, but that doesn't seem essential. We already run unit
tests across platforms and versions.

I chose to leave the code around for now in case somebody is using it,
but CI will no longer run it.