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.cpp

@@ -1467,7 +1467,11 @@ int ModelVolume::extruder_id() const
 
 bool ModelVolume::is_splittable() const
 {
-    return mesh.stl.stats.number_of_parts > 1;
+    // 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;
 }
 
 void ModelVolume::center_geometry()