Remove deprecated preprocessor function cleanup

[bouweandela]

Description

Remove deprecated preprocessor function cleanup. No upgrade action is available or required from users.

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[codecov]

Codecov Report

Merging #2215 (d2183e6) into main (837ce35) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2215      +/-   ##
==========================================
- Coverage   93.21%   93.21%   -0.01%     
==========================================
  Files         238      238              
  Lines       12851    12835      -16     
==========================================
- Hits        11979    11964      -15     
+ Misses        872      871       -1     

Files	Coverage Δ
esmvalcore/preprocessor/__init__.py	95.23% <ø> (-0.04%)	⬇️
esmvalcore/preprocessor/_io.py	86.44% <ø> (-0.36%)	⬇️

[schlunma]

Just one minor comment 👍

[valeriupredoi]

cheers, fellas 🍺

[valeriupredoi]

no need to squeakybumtime - it's not used anywhere in Tool