Bluetooth: Remove unused event mask struct

The struct for HCI_Set_Event_Mask is never used. Instead a local 8-byte
array is used for sending this command. Therefore, remove the
unnecessary struct definition.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>