delete system conntrack timeout custom ipv4 rule 1 destination port '53'
delete system conntrack timeout custom ipv4 rule 1 protocol udp replied '700'

vyos@vyos# commit
[ system conntrack ]
Failed to apply configuration: /run/nftables-ct.conf:20:21-21: Error:
syntax error, unexpected '}', expecting string     policy = { }

vyos@vyos# cat /run/nftables-ct.conf
#!/usr/sbin/nft -f


delete table ip vyos_conntrack
table ip vyos_conntrack {
    chain VYOS_CT_IGNORE {
         return
    }
    chain VYOS_CT_TIMEOUT {
        # rule-1 
        meta l4proto udp counter ct timeout set ct-timeout-1 comment "timeout-1"
        return
    }

    ct timeout ct-timeout-1 {
        l3proto ip;
        protocol udp;
        policy = {  }
    }
...

set system conntrack timeout custom ipv4 rule 1 destination port '53'
set system conntrack timeout custom ipv4 rule 1 protocol udp replied '700'
commit
delete system conntrack timeout custom ipv4 rule 1 destination port '53'
delete system conntrack timeout custom ipv4 rule 1 protocol udp replied '700'
commit