[cleanup] Remove code based on old basis and function interfaces

Both, the function space basis interface and the virtual function based grid/differentiable function interface have been deprecated for a long time and are now finally removed. As a consequnce all code for supporting this interface can also be removed.

There are two places where the old interfaces where used internally that could be replaced easily.

This supersedes !145 (closed).

mentioned in commit dc9be524

merged

mentioned in merge request !145 (closed)