Staging: hv: hv_mouse.c: remove struct mousevsc_drv_obj function callbacks

They aren't needed at all either because they are never called (OnOpen,
OnClose), or because we can just call the real function instead as it's
never set to anything else.

Just another step in unwinding the callback mess...

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