vsock: remove include/linux/vm_sockets.h file

This header file now only includes the "uapi/linux/vm_sockets.h".
We can include directly it when needed.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Jorgen Hansen <jhansen@vmware.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>