MNT: Remove dead code from APE 17

[pllim]

I think this stuff is left over from astropy-helpers days.

Also see https://github.com/astropy/astropy-APEs/blob/main/APE17.rst

[codecov]

Codecov Report

Merging #136 (7b0dce0) into main (7e7e4a6) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #136   +/-   ##
=======================================
  Coverage   70.09%   70.09%           
=======================================
  Files           9        9           
  Lines         602      602           
=======================================
  Hits          422      422           
  Misses        180      180           

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[tepickering]

it is indeed astropy-helpers cruft that i hadn't gotten around to cleaning up. there's more to do to clean up the package infrastructure here to bring it more inline with modern standards, but this is a good start. thanks!