Remove internal use warning for NHS constructors

[efaulhaber]

I moved this information to trixi-framework/TrixiParticles.jl#516.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@           Coverage Diff           @@
##             main       #4   +/-   ##
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  Coverage        ?   71.50%           
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  Files           ?        7           
  Lines           ?      200           
  Branches        ?        0           
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  Hits            ?      143           
  Misses          ?       57           
  Partials        ?        0           

Flag	Coverage Δ
unit	71.50% <ø> (?)

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