qemu: migration: Drop dead VNC cookie handling

qemu: migration: Drop dead VNC cookie handling

The only caller of this code is:

    for (i = 0; i < dom->def->ngraphics; i++) {
       if (dom->def->graphics[i]->type == VIR_DOMAIN_GRAPHICS_TYPE_SPICE) {
           if (!(mig->graphics =
                 qemuMigrationCookieGraphicsAlloc(driver, dom->def->graphics[i])))
               return -1;
           mig->flags |= QEMU_MIGRATION_COOKIE_GRAPHICS;
           break;
       }
    }

So this is never triggered for VNC, and in fact VNC has no support for
seamless migration anyways so that seems correct. Drop the dead VNC
handling.