patch 8.0.0620: checking for HAVE_GTK_MULTIHEAD is not needed

Problem:    Since we only support GTK versions that have it, the ckeck for
            HAVE_GTK_MULTIHEAD is no longer needed.
Solution:   Remove HAVE_GTK_MULTIHEAD. (Kazunobu Kuriyama)
diff --git a/src/gui_beval.c b/src/gui_beval.c
index 818a50b..abff1b0 100644
--- a/src/gui_beval.c
+++ b/src/gui_beval.c
@@ -1177,8 +1177,7 @@
 	int		x_offset = EVAL_OFFSET_X;
 	int		y_offset = EVAL_OFFSET_Y;
 	PangoLayout	*layout;
-# ifdef HAVE_GTK_MULTIHEAD
-#  if GTK_CHECK_VERSION(3,22,2)
+# if GTK_CHECK_VERSION(3,22,2)
 	GdkRectangle rect;
 	GdkMonitor * const mon = gdk_display_get_monitor_at_window(
 		gtk_widget_get_display(beval->balloonShell),
@@ -1187,17 +1186,13 @@
 
 	screen_w = rect.width;
 	screen_h = rect.height;
-#  else
+# else
 	GdkScreen	*screen;
 
 	screen = gtk_widget_get_screen(beval->target);
 	gtk_window_set_screen(GTK_WINDOW(beval->balloonShell), screen);
 	screen_w = gdk_screen_get_width(screen);
 	screen_h = gdk_screen_get_height(screen);
-#  endif
-# else
-	screen_w = gdk_screen_width();
-	screen_h = gdk_screen_height();
 # endif
 # if !GTK_CHECK_VERSION(3,0,0)
 	gtk_widget_ensure_style(beval->balloonShell);