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).

tests/fff_print/test_trianglemesh.cpp

@@ -342,7 +342,7 @@ SCENARIO( "TriangleMesh: Mesh merge functions") {
             cube.merge(cube2);
             cube.repair();
             THEN( "There are twice as many facets in the merged mesh as the original.") {
-                REQUIRE(cube.stl.stats.number_of_facets == 2 * cube2.stl.stats.number_of_facets);
+                REQUIRE(cube.facets_count() == 2 * cube2.facets_count());
             }
         }
     }