Remove custom fix for concatenation of aux factories now that bug in iris is solved

[schlunma]

Description

For at least 6 years, iris dropped derived coordinates (i.e., aux factories) during concatenation (see SciTools/iris#2478). For this reason, we implemented our own custom code to solve this:

ESMValCore/esmvalcore/preprocessor/_io.py

Line 39 in 1f73cc0

def _fix_aux_factories(cube):

Since the issue is solved now since iris 3.5, this PR removes the corresponding custom code in ESMValCore. I did not remove a tiny test that checks if the actual concatenation still works; and indeed, it does (also without our custom code) 🚀

Closes #2284

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.28%. Comparing base (d128753) to head (f8832e7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2392      +/-   ##
==========================================
- Coverage   94.29%   94.28%   -0.01%     
==========================================
  Files         246      246              
  Lines       13532    13511      -21     
==========================================
- Hits        12760    12739      -21     
  Misses        772      772              

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[valeriupredoi]

fastest review in history? Cheers, Manu! Bit of a shame to destroy so much good code 😭

[schlunma]

Well, the good code is now in iris, so I don't feel too bad about it 😄