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.