binder: remove unused BINDER_SMALL_BUF_SIZE define

It was never used since addition of binder to linux mainstream tree.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Arve Hjønnevåg" <arve@android.com>
Cc: Riley Andrews <riandrews@android.com>
Cc: devel@driverdev.osuosl.org
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>