Staging: hv: remove struct vmbus_channel_interface

Now that no one uses this structure, remove it, and channel_interface.c
(which is now empty) and the function, vmbus_get_interface() which is
not needed anymore either.

Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Hank Janssen <hjanssen@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>