Bluetooth: Kconfig: Get rid of BT_HCI_VS_EVT

Enabling vendor-specific extensions also implies support for vendor
events, so a separate Kconfig option for that is unnecessary.

One small additional thing this requires is the use of the
__maybe_unused annotation, since there's no-longer a single Kconfig
option that the controller hci.c can use to know that the vendor event
helper symbols are needed.

Signed-off-by: Johan Hedberg <johan.hedberg@gmail.com>