Remove max_links

[jorgensd]

Max_links was a way to give the kernel information about how many cells a single facet can be connected to.
We should rather let max_links=num_quadrature_points_per_facet and send in additional information to the assemblers as to which matrix each quadrature points relates to.

This would massively simplify the packing of test functions (and their corresponding gradients), and can be computed whenever we create the distance_map (which will be at packing).