Remove an invalid free on nodeIDs

[MakisH]

After merging #146, I started getting segfaults at the end of the simulation, in PreciceInterface_FreeData. The new calls added with #146 were:

free(preciceInterface->elemIPID);
free(preciceInterface->elemIPCoordinates);
free(preciceInterface->nodeIDs);
free(preciceInterface->elementsMeshName);

The issue here is that nodeIDs is not allocated, but a reference to previously allocated memory. There are two possible fixes:

A. Make the reference a copy, as suggested by Lucia in the comment that is still there, or
B. remove the invalid free.

The solution to (A) would be:

- interface->nodeIDs = &sim->ialset[sim->istartset[interface->nodeSetID] - 1]; // Lucia: make a copy
+  interface->nodeIDs   = malloc(interface->numNodes * sizeof(ITG));
+  getElementsIDs(interface->nodeSetID, sim->ialset, sim->istartset, sim->iendset, interface->nodeIDs);

but, since the current version has been working fine so far, I suggest just removing the invalid free, and not to introduce an unnecessary copy.