Remove: ZOOM_LVL_COUNT

This is the only enumeration with a COUNT and END. The logic of the COUNT
implied that BEGIN could be non-zero, but all but two uses of zoom level
assume that BEGIN is zero, making the separate count only confusing.

src/viewport.cpp

@@ -1477,7 +1477,7 @@ void ViewportSign::UpdatePosition(int center, int top, StringID str, StringID st
  */
 void ViewportSign::MarkDirty(ZoomLevel maxzoom) const
 {
-	Rect zoomlevels[ZOOM_LVL_COUNT];
+	Rect zoomlevels[ZOOM_LVL_END];
 
 	for (ZoomLevel zoom = ZOOM_LVL_BEGIN; zoom != ZOOM_LVL_END; zoom++) {
 		/* FIXME: This doesn't switch to width_small when appropriate. */