isisd: fix northbound circuit deletion

isisd: fix northbound circuit deletion

circuit deletion was being enforced by sending a fake IF_DOWN_FROM_Z
event for the circuit interface. This created a problem when the
circuit was enabled again, since isisd internal state machine was
expecting to see an IF_UP_FROM_Z that never came, as the interface
had not actually gone down.

As a consequence, disabling + re-enabling isis on an interface or
area would leave interfaces in a CONFIG state, and adjacencies were
not restored. Fix this by following the state machine and simply
disabling circuits rather than attempting to delete them forcefully.

Signed-off-by: Emanuele Di Pascale <emanuele@voltanet.io>