remove reference to psy-plot recipe in ESMValTool

[valeriupredoi]

Description

In ESMValGroup/ESMValTool#4063 we are retiring psy-plot recipe, so @schlunma prompted me to this particular reference in Core that also needs to be removed (many thanks, Manu!) 🍻
Docs: https://esmvaltool--2741.org.readthedocs.build/projects/ESMValCore/en/2741/

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.16%. Comparing base (d3eb1d1) to head (3ae589c).
⚠️ Report is 67 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2741   +/-   ##
=======================================
  Coverage   95.16%   95.16%           
=======================================
  Files         259      259           
  Lines       15171    15171           
=======================================
  Hits        14437    14437           
  Misses        734      734           

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[schlunma]

Cheers 🚀