BScrollBar: Remove DISABLE_ON_WINDOW_DEACTIVATION constant.

It was always enabled, and disabling it would break ABI.
If we want to make it disable-able, it needs to be a setting,
but I don't really see a reason for that.

src/kits/interface/ScrollBar.cpp

@@ -720,14 +720,12 @@ BScrollBar::MouseUp(BPoint where)
 }
 
 
-#if DISABLES_ON_WINDOW_DEACTIVATION
 void
 BScrollBar::WindowActivated(bool active)
 {
 	fPrivateData->fEnabled = active;
 	Invalidate();
 }
-#endif // DISABLES_ON_WINDOW_DEACTIVATION
 
 
 void