Bluetooth: GATT: Remove unused struct

prepare_write_data is no longer need since the ATT layer is now
responsible to queue the prepare writes using regular writes to commit
the data instead.

Change-Id: I8e35307a0489505b1475cdf31773c65a51165539
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>