Bluetooth: Mesh: don't send Friend Clear if Friend is lost

If LPN lost Friend node (all Poll attempts didn't succeed)
no reason to send Friend Clear to the Friend node.
This makes LPN powersupply  inefficient usage.
During Friend Clear sending LPN rejects any frame from
primary subnetwork.
Specificastion doesn't mandate that LPN would send Friend Clear
if polling didn't succeed.

Signed-off-by: Aleksandr Khromykh <aleksandr.khromykh@nordicsemi.no>