remove geweke diagnostic

[aloctavodia]

This removes geweke diagnostic as we have better diagnostic tools. I did not added a deprecation warning because geweke have been broken for a while and we have not heard any complaints, so probably nobody is using it anyways.

• 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 #1545 (8fbfa6d) into main (2dff682) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1545      +/-   ##
==========================================
- Coverage   91.09%   91.07%   -0.02%     
==========================================
  Files         105      105              
  Lines       11384    11361      -23     
==========================================
- Hits        10370    10347      -23     
  Misses       1014     1014              

Impacted Files	Coverage Δ
arviz/stats/__init__.py	100.00% <ø> (ø)
arviz/stats/diagnostics.py	99.21% <100.00%> (-0.05%)	⬇️

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[aloctavodia]

poor geweke, not even a little bit of resistant :-)