Revert "Use number_of_parts for is_splittable"

It is too dangerous to rely on the admesh flag without inspecting the
admesh code line by line and a through test.

This reverts commit cd3cec3e45.

src/libslic3r/Model.hpp

@@ -420,6 +420,12 @@ private:
     TriangleMesh             m_convex_hull;
     Geometry::Transformation m_transformation;
 
+    // flag to optimize the checking if the volume is splittable
+    //     -1   ->   is unknown value (before first cheking)
+    //      0   ->   is not splittable
+    //      1   ->   is splittable
+    mutable int               m_is_splittable{ -1 };
+
 	ModelVolume(ModelObject *object, const TriangleMesh &mesh) : mesh(mesh), m_type(ModelVolumeType::MODEL_PART), object(object)
     {
         if (mesh.stl.stats.number_of_facets > 1)