Notebook: Remove null check after dereference

This function clearly assumes the parameter children cannot be NULL, and
the call sites seem to perform enough checks to confirm this.

CID 1388869 (#1 of 1): Dereference before null check (REVERSE_INULL)
check_after_deref: Null-checking children suggests that it may be null,
but it has already been dereferenced on all paths leading to the check.

gtk/gtknotebook.c

@@ -6002,8 +6002,7 @@ gtk_notebook_calculate_tabs_allocation (GtkNotebook          *notebook,
     }
 
   /* Don't move the current tab past the last position during tabs reordering */
-  if (children &&
-      priv->operation == DRAG_OPERATION_REORDER &&
+  if (priv->operation == DRAG_OPERATION_REORDER &&
       direction == STEP_NEXT)
     {
       switch (tab_pos)