[StShadowTexture] Fix invalid unref of CoglHandle

This code appeared to treat the return value of clutter_texture_get_cogl_texture
as having a new reference, but in fact it doesn't.

https://bugzilla.gnome.org/show_bug.cgi?id=608119

src/st/st-shadow-texture.c

@@ -86,7 +86,6 @@ st_shadow_texture_create_shadow (StShadowTexture *st,
 
   cogl_texture_get_data (texture, COGL_PIXEL_FORMAT_A_8,
                          rowstride_in, pixels_in);
-  cogl_texture_unref (texture);
 
   if ((guint) st->blur_radius == 0)
     {