ospf6d: remove interface prefix when area is removed

ospf6d: remove interface prefix when area is removed

This bug was first reported in PR#7717. When removing an interface
from the area, the interface prefix is still shown

  r1# sh ipv6 ospf6 interface prefix
  *N IA 2013:12::/64       ::1      r1-r2-eth0 00:00:12

  r1# conf t
  r1(config)# router ospf6
  r1(config-ospf6)# no interface r1-r2-eth0 area 0.0.0.0
  r1(config-ospf6)# exit

  r1# sh ipv6 ospf6 interface prefix
  *N IA 2013:12::/64       ::1      r1-r2-eth0 00:00:22

This fix will check if the interface is disabled so the
prefix is not shown

Signed-off-by: ckishimo <carles.kishimoto@gmail.com>