Remove "callback" as a function argument to esmvalcore.preprocessor.load

[bouweandela]

Description

This removes the callback function argument from esmvalcore.preprocessor.load.

Closes #1800.

Link to documentation:

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[codecov]

Codecov Report

Merging #1801 (cb884e7) into main (4d48ece) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1801   +/-   ##
=======================================
  Coverage   91.51%   91.51%           
=======================================
  Files         202      202           
  Lines       10908    10909    +1     
=======================================
+ Hits         9982     9983    +1     
  Misses        926      926           

Impacted Files	Coverage Δ
esmvalcore/_recipe.py	95.81% <100.00%> (ø)
esmvalcore/preprocessor/_ancillary_vars.py	98.57% <100.00%> (ø)
esmvalcore/preprocessor/_io.py	84.73% <100.00%> (+0.06%)	⬆️
esmvalcore/preprocessor/_regrid.py	97.11% <100.00%> (ø)

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[bouweandela]

I'll deprecate it instead..