MNT: Cleanup of old unused code and configs

[bsipocz]

Some uncontroversial code cleanup.

This also removes unused config files, and the long-unused frontpage maker. @keflavich - I'll wait for your OK on this latter point before merging this.

[codecov]

Codecov Report

Merging #2231 (f12ddd0) into main (0833f63) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main    #2231   +/-   ##
=======================================
  Coverage   61.90%   61.91%           
=======================================
  Files         129      129           
  Lines       16310    16305    -5     
=======================================
- Hits        10097    10095    -2     
+ Misses       6213     6210    -3     

Impacted Files	Coverage Δ
astroquery/lamda/core.py	71.72% <0.00%> (+0.74%)	⬆️
astroquery/splatalogue/core.py	87.01% <ø> (+0.38%)	⬆️

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[keflavich]

is pip-requirements just handled by setup.cfg now?

rtd-environment is docs/rtd-something?

I'm just trying to catch up on ecosystem changes. The changes you propose are OK by me.

[bsipocz]

Yes, everything should be handled by setup.cfg now. RTD picks up the docs dependencies from the options.extras_require section, too (see #2229) while pip also works with that file automatically, so we don't need to maintain the dependency lists at multiple locations (well, we still need to update the narrative docs whenever we drop/add something, but at least there are no multiple config files floating around).