Simplify method 'ItemMaterialVariableBase::_initializeNewItems()' and cleanup code

[grospelliergilles]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 70.29%. Comparing base (35ea18f) to head (2b31b61).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...e/src/arcane/materials/ItemMaterialVariableBaseT.H	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1738      +/-   ##
==========================================
- Coverage   70.29%   70.29%   -0.01%     
==========================================
  Files        2257     2257              
  Lines      161822   161822              
  Branches    18651    18651              
==========================================
- Hits       113748   113745       -3     
- Misses      41322    41324       +2     
- Partials     6752     6753       +1     

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