MAINTAINERS: remove reference to bogus vsock file

The file drivers/vhost/vsock.h never existed.  Remove the bogus
MAINTAINERS reference.

Fixes: 433fc58e

 ("VSOCK: Introduce vhost_vsock.ko")
Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>