Simplify code by using cell->is_locally_owned() and friends

[kronbichler]

As far as I understand our code, statements like

          dof.get_triangulation().locally_owned_subdomain() ==
            numbers::invalid_subdomain_id ||
          cell->level_subdomain_id() ==
            dof.get_triangulation().locally_owned_subdomain()

is exactly what we do in cell->is_locally_owned_on_level(), see

dealii/include/deal.II/grid/tria_accessor.templates.h

Lines 3733 to 3739 in 4309760

	// Serial triangulations report invalid_subdomain_id as their locally owned
	// subdomain, so the first condition checks whether we have a serial
	// triangulation, in which case all cells are locally owned. The second
	// condition compares the subdomain id in the parallel case.
	return (this->tria->locally_owned_subdomain() ==
	numbers::invalid_subdomain_id ||
	this->level_subdomain_id() == this->tria->locally_owned_subdomain());

so use that shorter code. While there, I also observed that some code in MatrixFree used similar logic in a manual way, but we don't support any other than the partitioning implied by the Triangulation class anyway.

[peterrum]

is exactly what we do in cell->is_locally_owned_on_level(), see

Yes it is. I was annoyed that had to write these locs over and over again ;)

[bangerth]

If you find yourself annoyed by something, fix it :-)