Minor modification on PairedExplicitRK3 to reduce redundant computation

[warisa-r]

This PR removes redundant computations of c in TrixiNLsolveExt.jl for the current implementation of PairedExplicitRK3. Since c is already computed and passed to the function in the extension file, recalculating it is unnecessary.

This change also simplifies future adjustments where different sets of abscissae c might be needed.

Please let me know if there are any further improvements or changes needed!

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.36%. Comparing base (a0c14d7) to head (385de4e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2152   +/-   ##
=======================================
  Coverage   96.36%   96.36%           
=======================================
  Files         480      480           
  Lines       38069    38069           
=======================================
  Hits        36684    36684           
  Misses       1385     1385           

Flag	Coverage Δ
unittests	96.36% <100.00%> (ø)

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