xtables-monitor: Print -X command for base chains, too

xtables-monitor: Print -X command for base chains, too

Since commit 61e85e3192dea ("iptables-nft: allow removal of empty
builtin chains"), the command may be applied to "builtin" chains as
well, so the output is basically valid.

Apart from that, since kernel commit a1050dd07168 ("netfilter:
nf_tables: Reintroduce shortened deletion notifications") the base chain
deletion notification does not contain NFTNL_CHAIN_PRIO (actually:
NFTA_HOOK_PRIORITY) attribute anymore so this implicitly fixes for
changed kernel behaviour.

Signed-off-by: Phil Sutter <phil@nwl.cc>