Remove need for "self" link in MT trigger

So far we have been giving TCs an ElementLink called "self" upon creation which points to the object itself.

This was then used every time we needed to link the TC as a seed of another, via copying the "self" link & re-naming it as "seed".

We don't, however, need to do this as we can query the two pieces of information needed to construct the ElementLink directly from a Desision object when we want to link one as "seed" of another.

This saves some space as we don't need to have these extra "self" element links lying about in EDM classes.

CC @sgeorge, @tbold, @fpastore