Staging: hv: Get rid of vmbus_release_unattached_channels() as it is not used

Since vmbus_release_unattached_channels() is only used in module
unload path and since the vmbus driver cannot be unloaded,
get rid of this "dead" code.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Abhishek Kane <v-abkane@microsoft.com>
Signed-off-by: Hank Janssen <hjanssen@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>