remove default scale warning in compare

[OriolAbril]

loo and waic had it removed already IIRC

Checklist

• Follows official PR format
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[codecov]

Codecov Report

Merging #1412 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1412      +/-   ##
==========================================
- Coverage   91.69%   91.69%   -0.01%     
==========================================
  Files         105      105              
  Lines       11024    11023       -1     
==========================================
- Hits        10108    10107       -1     
  Misses        916      916              

Impacted Files	Coverage Δ
arviz/stats/stats.py	96.30% <ø> (-0.01%)	⬇️

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