Revert "Bluetooth: Controller: Fix ext conn create when using single timer"

This reverts commit a7fff744.

The next commit will re-add back the fix to primary
advertising channel plus auxiliary advertising channel too.
This will help keep the fix together regarding use of
single timer for software tIFS switching implementation.

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