Removed custom version of AtmosphereSigmaFactory

[schlunma]

Description

iris 3.1 added support for AtmosphereSigmaFactory (SciTools/iris#4052). For this reason we can remove our custom version of it.

Closes #1380 (Needs also changes in ESMValTool)

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[codecov]

Codecov Report

Merging #1382 (448435c) into main (6d9e4b9) will increase coverage by 0.22%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1382      +/-   ##
==========================================
+ Coverage   88.39%   88.61%   +0.22%     
==========================================
  Files         195      195              
  Lines        9958     9867      -91     
==========================================
- Hits         8802     8744      -58     
+ Misses       1156     1123      -33     

Impacted Files	Coverage Δ
esmvalcore/cmor/_fixes/cmip6/fgoals_g3.py	100.00% <ø> (ø)
esmvalcore/cmor/_fixes/shared.py	100.00% <ø> (+12.78%)	⬆️
esmvalcore/cmor/_fixes/cmip5/fgoals_g2.py	100.00% <100.00%> (ø)
esmvalcore/cmor/fixes.py	100.00% <100.00%> (ø)
esmvalcore/preprocessor/_io.py	86.55% <100.00%> (+2.10%)	⬆️

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[remi-kazeroni]

Looks good to me. Thanks for taking care of this @schlunma!