gh-93096: Remove python -m base64 -t

[arhadthedev]

test_base64.LegacyBase64TestCase.test_encodebytes/test_decodebytes is a more advanced version of the removed CLI argument.

The removed feature brings no benefit to an end user (unlike the rest of python -m base64).

• Issue: Decide the fate of undocumented script behavior of some modules #93096

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[corona10]

nit: Can we update usage to use f-string?

[arhadthedev]

Done. Should I go further and convert it to argparse?

[arhadthedev]

I have made the requested changes; please review again.

[bedevere-bot]

Thanks for making the requested changes!

@corona10: please review the changes made to this pull request.

[corona10]

lgtm