* atk/, MSVC_Net200[58]/atkmm/: Delete atkmm subdirectories.
* configure.ac: Remove module definition of atkmm-1.6.  Adjust the
pkg-config dependencies to pull in an installed atkmm.
* **/Makefile.am: Remove references to atkmm from the build rules.
* docs/reference/Doxyfile.in: Remove references to in-tree atkmm.
* gtk/{gtkmm,gtkmm-uninstalled}.pc.in: Remove atkmm substitutions.
* tools/extra_defs_gen/generate_defs_gtk.cc: Remove code to output
definitions for ATK types.