netfilter: remove redundant assignment to e

The assignment to variable e is redundant since the same assignment
occurs just a few lines later, hence it can be removed.  Cleans up
clang warning for arp_tables, ip_tables and ip6_tables:

warning: Value stored to 'e' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>