hw/hyperv/vmbus: Remove unused vmbus_load/save_req()

vmbus_save_req() and vmbus_load_req() are not used.
Remove them to avoid maintaining dead code.

This essentially reverts commit 4dd8a706
("vmbus: add infrastructure to save/load vmbus requests").

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211106134155.582312-2-philmd@redhat.com>
[MSS: Remove also corresponding variables, which are now unused]
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>