Bluetooth: controller: Remove redundant connection initiated check

Remove the redundant connection initiated check as the event
is closed on connection initiated and it is sufficient to
check in the prepare_cb function to abort any events in the
pipeline after the connection has been initiated.

Relates to commit 5ce5dc05 ("Bluetooth: controller:
Avoid race between ULL and LLL when initiating conn") and
commit 18f5fb99 ("Bluetooth: controller: Remove use of
lll_stop").

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