remove ess_mean and ess_sd from summary

[aloctavodia]

• Follows official PR format
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[codecov]

Codecov Report

Merging #1539 (5e5015a) into main (2be3611) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1539   +/-   ##
=======================================
  Coverage   91.09%   91.09%           
=======================================
  Files         105      105           
  Lines       11382    11382           
=======================================
  Hits        10368    10368           
  Misses       1014     1014           

Impacted Files	Coverage Δ
arviz/stats/diagnostics.py	99.26% <100.00%> (ø)
arviz/stats/stats.py	96.12% <100.00%> (ø)

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