operations/core/nop: Remove a needless and wrong cast

g_object_ref accepts a gpointer, not a GObject *, and since we have
already cast it to a GeglBuffer there is not much to be had in the way
of sanity checking either. We could have cast it to a gpointer, but it
is idiomatic to not do so.

operations/core/nop.c

@@ -61,7 +61,7 @@ gegl_nop_process (GeglOperation        *operation,
       return FALSE;
     }
 
-  gegl_operation_context_take_object (context, "output", g_object_ref (G_OBJECT (input)));
+  gegl_operation_context_take_object (context, "output", g_object_ref (input));
   return TRUE;
 }