Remove support for multiple configurations in one file

[stsewd]

close #4705

[stsewd]

I still need to do a manual check, but I believe this is ready

[codecov]

Codecov Report

Merging #4800 into master will decrease coverage by 0.04%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master    #4800      +/-   ##
==========================================
- Coverage   76.22%   76.18%   -0.05%     
==========================================
  Files         158      158              
  Lines       10025    10014      -11     
  Branches     1264     1261       -3     
==========================================
- Hits         7642     7629      -13     
- Misses       2039     2040       +1     
- Partials      344      345       +1

Impacted Files	Coverage Δ
readthedocs/doc_builder/config.py	91.89% <ø> (ø)	⬆️
readthedocs/config/config.py	98% <100%> (-0.04%)	⬇️
readthedocs/config/parser.py	87.5% <80%> (-12.5%)	⬇️

[stsewd]

I tested this locally, nothing breaks :)

[ericholscher]

Looks good to me. I don't fully have the config code in my head, but I think this makes things simpler, which is great.

[ericholscher]

Let's go ahead and merge this and get it tested locally. I don't think it should effect anyone in prod, but if so we can deal with it when that happens. 🎉