[DF] Remove `InvalidRef` test case: it does not happen anymore
[DF] Remove `InvalidRef` test case: it does not happen anymore

With the new ownership model a head node that goes out of scope in
user code is still valid. Remove a test for an exception that is not
thrown anymore.

olifre/root,karies/root,karies/root,olifre/root,karies/root,olifre/root,karies/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,karies/root,olifre/root,karies/root,karies/root,karies/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,karies/root,root-mirror/root,olifre/root