Bluetooth: 6lowpan: Enable delete_netdev to be scheduled when last peer is deleted

This patch fixes an issue with the netdev not being unregistered when
the last peer is deleted. Removing the logical negation operator on the
boolean solves this issue. If the last peer is removed the condition
will be true, and the delete_netdev() is scheduled.

Signed-off-by: Lukasz Duda <lukasz.duda@nordicsemi.no>
Signed-off-by: Glenn Ruben Bakke <glenn.ruben.bakke@nordicsemi.no>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>