Remove nonfunctional bin/towncrier

[hynek]

Obviously, nobody has been using this pre-entrypoint gunk for years, because it hasn't worked since we renamed _main.py to __main__.py.

Couldn't find any mentions in the docs either, so this can safely go.

Checklist

• Make sure changes are covered by existing or new tests.
• For at least one Python version, make sure test pass on your local environment.
• Create a file in src/towncrier/newsfragments/. Briefly describe your
  changes, with information useful to end users. Your change will be included in the public release notes.
• Make sure all GitHub Actions checks are green (they are automatically checking all of the above).
• Ensure docs/tutorial.rst is still up-to-date.
• If you add new CLI arguments (or change the meaning of existing ones), make sure docs/cli.rst reflects those changes.
• If you add new configuration options (or change the meaning of existing ones), make sure docs/configuration.rst reflects those changes.

[adiroiban]

Thanks for the cleanup.