Remove wShowGNUstepPanel() etc

The only place where this function is called is from a double click
in the first icon of the dock, and only if there's no program already
associated with it.

This is a bit superfluous and most people have defined the first
icon to call WPrefs instead and end up never seeing that panel.
And since the last commit ("Change behaviour of the GNUstep dockapp"),
this is now also the default behaviour of Window Maker.

Furthermore, the panel itself is not accurate. Window Maker is not part
of the GNUstep project.

src/dialog.h

@@ -41,11 +41,7 @@ int wExitDialog(WScreen *scr, char *title, char *message, char *defBtn,
 Bool wIconChooserDialog(WScreen *scr, char **file, char *instance, char *class);
 
 void wShowInfoPanel(WScreen *scr);
-
 void wShowLegalPanel(WScreen *scr);
-
-void wShowGNUstepPanel(WScreen *scr);
-
 int wShowCrashingDialogPanel(int whatSig);