Don't account for size grip under platforms not showing it

Size grip in generic wxStatusBar is only supported in wxGTK as it's
drawn using GTK functions, but this is not really a problem as other
platforms either use native implementations (MSW, Qt) or shouldn't show
size grip anyhow as it looks non-native (Mac).

So just ensure we don't leave space for the grip if it's not shown,
correcting the change of 6c1b2b23cf.

Closes #18469.

include/wx/generic/statusbr.h

@@ -81,7 +81,7 @@ protected:
 
     // true if the status bar shows the size grip: for this it must have
     // wxSTB_SIZEGRIP style and the window it is attached to must be resizable
-    // and not maximized
+    // and not maximized (note that currently size grip is only used in wxGTK)
     bool ShowsSizeGrip() const;
 
     // returns the position and the size of the size grip