libgimpconfig: don't unref the GFile in gimp_config_serialize_value()

g_value_get_object() doesn't return a new reference, so it must
not be dropped. Fixes random crashes when a profile was chosen
in a GimpTemplate (e.g. in new image or prefs). Spotted by Elle.

libgimpconfig/gimpconfig-serialize.c

@@ -499,7 +499,6 @@ gimp_config_serialize_value (const GValue *value,
             g_string_append (str, parse_name);
 
           g_free (parse_name);
-          g_object_unref (file);
         }
       else
         {