Bluetooth: Mesh: Remove extra rescheduling and ADV_FLAG_SCHEDULED in adv

This commit removes extra rescheduling in the extended advertiser after
the last advertising has been finished. This commit also removes the
`ADV_FLAG_SCHEDULED` flag as not needed anymore.

This extra rescheduling and the flag were needed to delay the next
advertisement by 20ms. Now, when the delay is removed the advertiser can
pick the next advertisement from the pool and push to the host right
away.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>