Include sample_z in _cleanup_samples()

[Micki-D]

This fixes a bug that arises when '_cleanup_samples()' is called when the last proposed sample in the chain has been accepted.
This would cause the samples stored in the chain_state to be cleaned up, but the sample_z would remain in the old state, causing faulty subsequent steps.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.51%. Comparing base (4d2f76b) to head (a7662fe).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #462      +/-   ##
==========================================
+ Coverage   49.47%   49.51%   +0.04%     
==========================================
  Files         121      121              
  Lines        5807     5812       +5     
==========================================
+ Hits         2873     2878       +5     
  Misses       2934     2934              

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[oschulz]

Thanks!