UEqn: Removed "Sp" term from the momentum equation of the steady-state solvers.

It is not clear that this "correction" term has provided any benefit and there
are cases for which it has proved detrimental to stability and convergence.

tutorials/incompressible/MRFSimpleFoam/MRFSimpleFoam/MRFSimpleFoam.C

@@ -64,7 +64,6 @@ int main(int argc, char *argv[])
             tmp<fvVectorMatrix> UEqn
             (
                 fvm::div(phi, U)
-              - fvm::Sp(fvc::div(phi), U)
               + turbulence->divDevReff(U)
             );
             mrfZones.addCoriolis(UEqn());