Removed the PRUS format parser. WIP: admesh eradication:

stl_stats are newly only accessed by TriangleMesh::stats(),
most of the direct access to TriangleMesh::stl is gone with the exception
of parsing input files (3MF, AMF, obj).

src/slic3r/GUI/3DScene.hpp

@@ -128,6 +128,8 @@ public:
     void load_mesh(const TriangleMesh& mesh) { this->load_mesh_full_shading(mesh); }
 #endif // ENABLE_SMOOTH_NORMALS
 
+    void load_its_flat_shading(const indexed_triangle_set &its);
+
     inline bool has_VBOs() const { return vertices_and_normals_interleaved_VBO_id != 0; }
 
     inline void reserve(size_t sz) {