Bluetooth: Controller: Remove redundant EVENT_IFS_US in BIS timing

Remove redundant EVENT_IFS_US used in Control PDU timing
calculation. The BIG radio event ends with Control Subevent
and hence there is no need to add EVENT_IFS_US at the end
of the BIG event.

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