netfilter: nf_tables: No need to check chain existence when tracing

nft_trace_notify() is called only from __nft_trace_packet(), which
assigns its parameter 'chain' to info->chain. __nft_trace_packet() in
turn later dereferences 'chain' unconditionally, which indicates that
it's never NULL. Same does nft_do_chain(), the only user of the tracing
infrastructure. Hence it is safe to assume the check removed here is not
needed.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>