vhost: Remove unused vhost_dev_{load|save}_inflight

vhost: Remove unused vhost_dev_{load|save}_inflight

vhost_dev_load_inflight and vhost_dev_save_inflight have been
unused since they were added in 2019 by:

5ad204bf2a ("vhost-user: Support transferring inflight buffer between qemu and backend")

Remove them, and their helper vhost_dev_resize_inflight.

Signed-off-by: Dr. David Alan Gilbert <dave@treblig.org>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>