Sculpt: Allow non-manifold collapse

* This was always buggy, which is why
  I have code that detects and snips off
  non-manifold fins.
* Turns out it is more robust to simply allow
  non-manifold collapses and clean up geometry
  afterwards.  IIRC this is how DynTopo (and the
  paper it's based off of) originall worked.

source/blender/blenkernel/intern/dyntopo.c

@@ -3739,7 +3739,7 @@ static void pbvh_bmesh_collapse_edge(PBVH *pbvh,
   }
 
   if (bm_edge_collapse_is_degenerate_topology(e)) {
-    return;
+    // return;
   }
 
   pbvh_check_vert_boundary(pbvh, v1);