Get rid of Sys_DoPreferences()

The OSX backend was the only one utilizing this to set CVars
for the video mode.
Not required anymore since its now based on SDL.

CMakeLists.txt

@@ -623,8 +623,6 @@ if (APPLE)
 	set(src_sys_core
 		sys/glimp.cpp
 		sys/glimp_ati_fragment_shader.cpp
-		sys/osx/PickMonitor.cpp
-		sys/osx/PreferencesDialog.cpp
 		sys/osx/DOOMController.mm
 		sys/osx/macosx_misc.mm
 	)

sys/sys_public.h

@@ -247,9 +247,6 @@ int				Sys_ListFiles( const char *directory, const char *extension, idList<class
 // know early if we are performing a fatal error shutdown so the error message doesn't get lost
 void			Sys_SetFatalError( const char *error );
 
-// display perference dialog
-void			Sys_DoPreferences( void );
-
 /*
 ==============================================================