Use number_of_parts for is_splittable

It's there, why not use it

src/libslic3r/Model.cpp

@@ -1467,11 +1467,7 @@ int ModelVolume::extruder_id() const
 
 bool ModelVolume::is_splittable() const
 {
-    // the call mesh.is_splittable() is expensive, so cache the value to calculate it only once
-    if (m_is_splittable == -1)
-        m_is_splittable = (int)mesh.is_splittable();
-
-    return m_is_splittable == 1;
+    return mesh.stl.stats.number_of_parts > 1;
 }
 
 void ModelVolume::center_geometry()

src/libslic3r/Model.hpp

@@ -420,12 +420,6 @@ 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)

src/libslic3r/TriangleMesh.cpp

@@ -338,19 +338,6 @@ void TriangleMesh::rotate(double angle, Point* center)
     this->translate(c(0), c(1), 0);
 }
 
-/**
- * Calculates whether or not the mesh is splittable.
- */
-bool TriangleMesh::is_splittable() const
-{
-    std::vector<bool> visited;
-    find_unvisited_neighbors(visited);
-
-    // Try finding an unvisited facet. If there are none, the mesh is not splittable.
-    auto it = std::find(visited.begin(), visited.end(), false);
-    return it != visited.end();
-}
-
 /**
  * Visit all unvisited neighboring facets that are reachable from the first unvisited facet,
  * and return them.

src/libslic3r/TriangleMesh.hpp

@@ -68,7 +68,6 @@ public:
     size_t facets_count() const { return this->stl.stats.number_of_facets; }
     bool   empty() const { return this->facets_count() == 0; }
 
-    bool is_splittable() const;
     std::deque<uint32_t> find_unvisited_neighbors(std::vector<bool> &facet_visited) const;
 
     stl_file stl;