isisd: don't remove interface config when isis router is deleted

isisd: don't remove interface config when isis router is deleted

In previous releases, it was not possible to configure ISIS on an
interfaces without configuring the ISIS router first. Therefore, we had
to delete the ISIS config from all interfaces when the router config was
deleted. This is fixed since version 8.0 – interface and router configs
are completely separate and don't depend on each other, so now we can
remove this hack and preserve the interface config when the router
config is deleted.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>