[cleanup] Remove 'sequential' code. Support is discontinued after release 3.4.

dumux/common/boundaryconditions.hh

dumux/common/start.hh

dumux/common/timemanager.hh

dumux/io/vtkmultiwriter.hh

dumux/io/vtknestedfunction.hh

dumux/material/spatialparams/sequentialfv.hh

dumux/material/spatialparams/sequentialfv1p.hh

dumux/porousmediumflow/1p/CMakeLists.txt

dumux/porousmediumflow/1p/sequential/CMakeLists.txt

dumux/porousmediumflow/1p/sequential/celldata.hh

...x/porousmediumflow/1p/sequential/diffusion/CMakeLists.txt

...mflow/1p/sequential/diffusion/cellcentered/CMakeLists.txt

...diumflow/1p/sequential/diffusion/cellcentered/pressure.hh

...p/sequential/diffusion/cellcentered/pressureproperties.hh

.../1p/sequential/diffusion/cellcentered/pressurevelocity.hh

...diumflow/1p/sequential/diffusion/cellcentered/velocity.hh

dumux/porousmediumflow/1p/sequential/diffusion/problem.hh

dumux/porousmediumflow/1p/sequential/diffusion/properties.hh

dumux/porousmediumflow/1p/sequential/fluxdata.hh

dumux/porousmediumflow/1p/sequential/indices.hh