virtio-console: no need to remove char handlers explicitly

virtio-console: no need to remove char handlers explicitly

qdev is now equipped (thanks to the last commit) to disassociate
chardevs from the qdev devices on the devices going away.  So doing it
in the virtio-console driver is not necessary.

Since that was the only thing being done in the qdev exit method, drop
it entirely.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>