Small fixes of linalg.decompositions

[babacry]

• Remove unused code.
• Actual raise in kron 4x4 to 2x2s (raise was in docstring, but not actualized). Update related tests as well.
• Other nit fixes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.17%. Comparing base (78d30e7) to head (45db532).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7128   +/-   ##
=======================================
  Coverage   98.16%   98.17%           
=======================================
  Files        1093     1093           
  Lines       95420    95441   +21     
=======================================
+ Hits        93665    93695   +30     
+ Misses       1755     1746    -9     

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