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/libslic3r/CMakeLists.txt

@@ -78,8 +78,6 @@ add_library(libslic3r STATIC
     Format/OBJ.hpp
     Format/objparser.cpp
     Format/objparser.hpp
-    Format/PRUS.cpp
-    Format/PRUS.hpp
     Format/STL.cpp
     Format/STL.hpp
     Format/SL1.hpp