Implement wxColourDisplay() in terms of wxDisplay::GetDepth()

Get rid of another global display-related function, even though this one
only had non-trivial implementation under MSW.

src/motif/utils.cpp

@@ -225,12 +225,6 @@ void wxGetMousePosition( int* x, int* y )
 #endif
 }
 
-// Return true if we have a colour display
-bool wxColourDisplay()
-{
-    return wxDisplayDepth() > 1;
-}
-
 void wxDisplaySizeMM(int *width, int *height)
 {
     Display *dpy = wxGlobalDisplay();