vmbus: remove no longer used signal_policy

The explicit signal policy is no longer used. A different mechanism
will be added later when xmit_more is supported.

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