[PL] Remove unused coupled_xs computeSecondaryVars

ProcessLib/HeatConduction/HeatConductionProcess.cpp

ProcessLib/HeatTransportBHE/HeatTransportBHEProcess.cpp

ProcessLib/HydroMechanics/HydroMechanicsProcess.cpp

ProcessLib/LIE/SmallDeformation/SmallDeformationProcess.cpp

ProcessLib/LiquidFlow/LiquidFlowProcess.cpp

ProcessLib/LocalAssemblerInterface.cpp

ProcessLib/LocalAssemblerInterface.h

ProcessLib/RichardsMechanics/RichardsMechanicsProcess.cpp

ProcessLib/SmallDeformation/SmallDeformationProcess.cpp

...sLib/ThermoHydroMechanics/ThermoHydroMechanicsProcess.cpp