Remove stergm() and related utilities?

[krivit]

It's been deprecated since 2022, and there is no point in maintaining it anymore.

I could leave it in indefinitely, with a prominent help message that the code is unmaintained and if it stops working, it will be removed rather than fixed?

@martinamorris , @sgoodreau ?

[krivit]

@smjenness ?

[sgoodreau]

I'm fine with any option.