Bluetooth: GATT: Remove obsolete flush callback

It is no longer necessary to implement flush callback for long
descriptors since the stack can queue prepare writes this callback
will never be called which makes BT_GATT_LONG_DESCRIPTOR obsolete
as well.

Change-Id: Idca31ba8e4404d2acba760c420394d5adee0a508
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>