netfilter: nftables: remove redundant assignment of variable err

The variable err is being assigned a value that is never read,
the same error number is being returned at the error return
path via label err1.  Clean up the code by removing the assignment.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>