[freeflow] Remove unnecessary phaseIdx as function arguments

dumux/discretization/staggered/freeflow/fickslaw.hh

dumux/discretization/staggered/freeflow/maxwellstefanslaw.hh

dumux/freeflow/navierstokes/volumevariables.hh

dumux/freeflow/navierstokesnc/staggered/fluxvariables.hh

dumux/freeflow/navierstokesnc/staggered/localresidual.hh

dumux/freeflow/navierstokesnc/volumevariables.hh

dumux/freeflow/navierstokesnc/vtkoutputfields.hh

dumux/freeflow/rans/volumevariables.hh

dumux/freeflow/ransnc/volumevariables.hh

test/freeflow/navierstokesnc/msfreeflowtestproblem.hh