No longer store dofs_per_vertex for each MG vertex.

No longer store dofs_per_vertex for each MG vertex.

In #4564, I already noted that the way we store vertex DoF indices in the MG
case was not particularly efficient because we kept around a dynamically
sized 'offset' array for each vertex whose entries were computable: they were
successive multiples of dofs_per_cell. So #4564 replaces that by just storing
dofs_per_cell itself for each vertex.

But that, of course, is also wasteful: why store it once per vertex when that
will be the same number for every vertex. Rather, what this patch does is
to pass that number to the functions that access these DoF indices, so that
we can use the global fe.dofs_per_vertex instead. This saves 4 bytes per
vertex, and maybe a few cycles of CPU time on top of it.