Bluetooth: GATT: Remove handles from macros

bt_gatt_register will assign a handle if not initialized and removing
the handle makes it simpler to change the attributes since it is no
longer possible to have conflicting handles.

Change-Id: I787f7325cc990c360056b1aefd07bb7d7876b445
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>