Weed out unused utils functions

[teutoburg]

Remove utils functions that were not used anywhere in the codebase (I also checked our other main packages).

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 74.94%. Comparing base (8f8963e) to head (afa9616).

Files	Patch %	Lines
scopesim/utils.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           dev_master     #381      +/-   ##
==============================================
+ Coverage       74.24%   74.94%   +0.69%     
==============================================
  Files              56       56              
  Lines            7867     7770      -97     
==============================================
- Hits             5841     5823      -18     
+ Misses           2026     1947      -79     

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[teutoburg]

We have two incarnations of zenith distance to airmass (and vice-versa) conversion functions each. The implementations are (shockingly) identical, but the names and level of docstring details differ. Also, only one variant of each is actually used anywhere. However, I slightly prefer the name of the one that is not used and less well documented, mostly for readability. I might just end up replacing the names with the underscore ones, and keeping the variant that's better documented. But for now I just put them next to each other, so it's at least more obvious that we have this duplication (they used to be half a module apart, so I guess the duplication was missed until now).

Also, diff coverage is complaining because I moved the unused (and untested) variants of these functions...

[teutoburg]

Docs build fails (also after retry) with another ContentTooShortError, which I think is unrelated.

[hugobuddel]

These functions being unused and untested is reason enough to remove them, because now they are a liability, not an asset.

About the airmass function names, both names are bad. Maybe I should add the functions zenith_dist_from_airmass() and airmass_from_zenith_dist() to this file 😜!