Staging: lustre: obd_support.h: Remove unused OBD_SLAB_FREE_RTN0

Remove macro OBD_SLAB_FREE_RTN0 since it is defined but not used

Signed-off-by: Shraddha Barke <shraddha.6596@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>