Remove unused Solver::newton_maxlinsteps_

[dweindl]

Left over from #1732.

Closes #2590.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.63%. Comparing base (7ba2165) to head (a339089).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2592   +/-   ##
========================================
  Coverage    77.63%   77.63%           
========================================
  Files          329      329           
  Lines        22061    22061           
  Branches      1456     1456           
========================================
  Hits         17126    17126           
  Misses        4924     4924           
  Partials        11       11           

Flag	Coverage Δ
cpp	73.56% <ø> (ø)
cpp_python	34.30% <ø> (ø)
petab	37.23% <ø> (ø)
python	72.39% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
include/amici/solver.h	94.11% <ø> (ø)

---- 🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests