[staggered] Connectivity map stores own dofIdx

This is not needed, as the own dof is always known.

Concerns the staggered local assembler, probably the matrix pattern helper and the elementvolvars.

I've encountered this in the Staggered Higher Order work as well. I can try fixing this in the newer version.

assigned to @nedc and unassigned @kweis

mentioned in merge request !1518 (merged)

mentioned in merge request !1524 (merged)

mentioned in commit 929be909

closed via merge request !1524 (merged)

mentioned in issue #910 (closed)