plug-ins: Remove unneeded gegl_buffer_iterator_add()

Resolves #5553
As noted by Massimo, in the Sparkle plug-in there is an
extra gegl_buffer_iterator_add () that is not used in the following
gegl_buffer_iterator_next (iter) loop. Since this adds dest_buffer
which is also written to in the loop in the fspike () functions,
it can cause data to be overwritten. This patch removes the
unneeded extra function call.

plug-ins/common/sparkle.c

@@ -746,11 +746,6 @@ sparkle (GObject      *config,
                                    format,
                                    GEGL_ACCESS_READ, GEGL_ABYSS_NONE, 2);
 
-  gegl_buffer_iterator_add (iter, dest_buffer,
-                            GEGL_RECTANGLE (x1, y1, width, height), 0,
-                            format,
-                            GEGL_ACCESS_WRITE, GEGL_ABYSS_NONE);
-
   while (gegl_buffer_iterator_next (iter))
     {
       GeglRectangle  roi = iter->items[0].roi;