Remove crosslink, salt and further unused code from polymer.?pp

[RudolfWeeber]

Fixes #

Description of changes:

PR Checklist

• Tests?
  • Interface
  • Core
• Docs?

[codecov]

Codecov Report

Merging #1793 into python will increase coverage by 1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           python   #1793     +/-   ##
========================================
+ Coverage      58%     59%     +1%     
========================================
  Files         397     417     +20     
  Lines       27372   29043   +1671     
========================================
+ Hits        15978   17248   +1270     
- Misses      11394   11795    +401

Impacted Files	Coverage Δ
src/core/polymer.cpp	27% <ø> (+11%)	⬆️
src/core/EspressoSystemInterface.hpp	41% <0%> (-22%)	⬇️
src/core/lees_edwards.cpp	81% <0%> (-19%)	⬇️
src/core/lb.cpp	63% <0%> (-7%)	⬇️
src/core/grid.hpp	81% <0%> (-7%)	⬇️
src/core/thermostat.hpp	93% <0%> (-5%)	⬇️
src/core/reaction_field.hpp	11% <0%> (-3%)	⬇️
src/core/iccp3m.cpp	73% <0%> (-3%)	⬇️
src/core/constraints/ShapeBasedConstraint.cpp	30% <0%> (-2%)	⬇️
src/core/integrate.cpp	73% <0%> (-2%)	⬇️
... and 122 more

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