Revert "Check face graph validity and not just halfedge validity in post-reading BGL IO"

This reverts commit 226f84f2e6.

This is too strong and prevents reading some broken meshes that we still
want to be able to read.

Note that even is_valid_halfedge_graph() would return false

BGL/include/CGAL/boost/graph/IO/Generic_facegraph_builder.h

@@ -151,7 +151,7 @@ public:
         put(fcm, f, face_colors[i]);
     }
 
-    return is_valid_face_graph(g);
+    return is_valid(g);
   }
 
 protected: