Remove ATSU-related dead code from wxOSX

ATSU was deprecated for a long time and was finally removed from OS X
SDK in 10.11. wxOSX_USE_ATSU_TEXT was unconditionally disabled already,
so the code was never used and can be safely removed.

include/wx/osx/cocoa/chkconf.h

@@ -26,12 +26,6 @@
     #define wxOSX_USE_NATIVE_FLIPPED 1
 #endif
 
-/*
- * text rendering system
- */
-
-#define wxOSX_USE_ATSU_TEXT 0
-
 /*
  * Audio System
  */