Bluetooth: controller: Revert strict preempt ticker start failure check

Revert the strict preempt ticker start failure check.
Preempt ticker start can fail when enqueuing prepares into
already filled pipeline which has preempt ticker already
started for the first prepare that was added in the
pipeline.

Regression introduced in commit 5b75bdf5 ("Bluetooth:
controller: nRF5: Check preempt event on timeout").

Fixes #35476.

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