Tidy Set_Rhs_Magnitude function and remove unused function `v_NekSysInitialGuess`

CHANGELOG.md

library/LibUtilities/LinearAlgebra/NekLinSysIter.cpp

library/LibUtilities/LinearAlgebra/NekLinSysIter.h

library/LibUtilities/LinearAlgebra/NekLinSysIterCG.cpp

library/LibUtilities/LinearAlgebra/NekLinSysIterCG.h

...ibUtilities/LinearAlgebra/NekLinSysIterFixedpointJacobi.h

library/LibUtilities/LinearAlgebra/NekLinSysIterGMRES.cpp

library/LibUtilities/LinearAlgebra/NekLinSysIterGMRES.h

library/LibUtilities/LinearAlgebra/NekLinSysIterGMRESLoc.h

library/LibUtilities/LinearAlgebra/NekNonlinSys.cpp

library/LibUtilities/LinearAlgebra/NekNonlinSysNewton.cpp

library/LibUtilities/LinearAlgebra/NekNonlinSysNewton.h

library/LibUtilities/LinearAlgebra/NekSys.cpp

library/LibUtilities/LinearAlgebra/NekSys.h