Retire: py-sendmail-admin and py-boto

[Adam <adam%netbsd.org@localhost>]

I would like to remove

* mail/py-sendmail-admin - does not work with Python 3
* net/py-boto - does not work with Python 3.12+

These are very old, unmaintained upstream, and no other package depend on them.

If there are no objections, I will remove them in a week.

Kind regards,
Adam