Drivers: hv: vmbus: Introduce a function to remove a rescinded offer

In response to a rescind message, we need to remove the channel and the
corresponding device. Cleanup this code path by factoring out the code
to remove a channel.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>