Remove useless shebangs in non-executable files (#2161)

Such shebangs are only ever used if the file is executed directly, i.e.:

    $ /usr/lib/python3.9/site-packages/black_primer/cli.py

But that doesn't work:

    $ /usr/lib/python3.9/site-packages/black_primer/cli.py
    bash: /usr/lib/python3.9/site-packages/black_primer/cli.py: Permission denied

The lib file even has: "lib is a library, funnily enough"