Bluetooth: Audio: Remove PA sync handling from scan_delegator

The scan delegator will no longer be responsible for
handling the actual synchronization of the periodic
advertisers, and will offload this to higher layers.

The reason for this, is that scanning is a global state,
and should be avoided autonousmly by the stack. The
application is much better suited for handling this.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>