arptables: Fix jumps into user-defined chains

arptables: Fix jumps into user-defined chains

Trying to jump into a user-defined chain was not possible:

| arptables-nft -N foo
| arptables-nft -A INPUT -j foo
| (null) v1.8.0 (nf_tables):  RULE_APPEND failed (No such file or directory): rule in chain INPUT

Since nft_arp_add() already does the right thing if cs->target is NULL
and cs->jumpto contains a non-empty string, simply drop the block of
code trying to deal with the situation.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>