ospfd: reset mpls-te prior to ospf router removal

ospfd: reset mpls-te prior to ospf router removal

This commits attempts to fix a problem that occurs when mpls-te gets
removed from ospfd config. Mpls-te has an inter-as option, which can be
set to Off/Area/AS. Whenever the inter-as takes "Area" or "AS" as a
value, this value will not be cleaned after removing mpls-te or after
removing the ospf router. Therefore, if mpls-te is configured with
inter-as AS or Area and we remove mpls-te or the ospf router, the
inter-as will still preserve its value; therefore, next time mpls-te is
enabled, it will automatically inherits the previous inter-as value
(either Area or AS). This leads to wrong configuration, which can be a
problem for frr_reload.py.
The commits forces mpls-te to reset inter-as to Off before it mpls-te
gets removed from the configuration and before the ospf router gets
removed.

Signed-off-by: Carlo Galiotto <carlo@voltanet.io>