Do not set bend points on elk edge when transforming from Sprotty edge

[jbicker]

Fixes #427

The bend points are now completely neglected in the edge transformation from SEdge to an ElkExtendedEdge. So elk sets completely new bend points.

Feels a bit like a naive fix but it works at least for the random graph example. I hope it does not break something else.

[spoenemann]

In some cases, an ELK layout algorithm might use the previous bend points in its computations, so we shouldn't remove them. Could you try to fix it in the other direction, making sure that bend points are erased in the SModel when ELK doesn't specify any?

[jbicker]

@spoenemann, thanks for the hint. I did as you proposed.

[spoenemann]

Hmmm, it seems to me that the root of the problem is somewhere else. The elkjs library uses this ELK Java/Xtend class method to transfer the layout algorithm results to the JSON data:
https://github.com/kieler/elkjs/blob/72b471e65f5c5d7b0e94450b14c14bdf12384f82/src/java/org/eclipse/elk/js/ElkJs.java#L206

I created eclipse-elk/elk#1001 to discuss and possibly fix this in ELK.

[jbicker]

Could this PR then be closed?

[spoenemann]

I think so yes. The current solution smells like a workaround for a problem that lies somewhere else.