Bluetooth: controller: Remove is_enabled flag for BIS events

Remove is_enabled flag in LLL of BIS events context, as the
status returned from ticker_stop interface is sufficient to
determine if the Broadcast ISO Sync was established or not.
HCI Command Disallow as reason is to be returned if sync was
not established or sync was loss prior to call of HCI LE
BIG Terminate Command was called.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>