ionic: stop watchdog timer earlier on remove

We need to be better at making sure we don't have a link check
watchdog go off while we're shutting things down, so let's stop
the timer as soon as we start the remove.

Meanwhile, since that was the only thing in
ionic_dev_teardown(), simplify and remove that function.

Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>