netfilter: cttimeout: remove 'l3num' attr check

After commit dd2934a9 ("netfilter: conntrack: remove l3->l4 mapping
information"), the attribute of type `CTA_TIMEOUT_L3PROTO` is not used
any more in function cttimeout_default_set.

However, the previous commit ea9cf2a5 ("netfilter: cttimeout: remove
set but not used variable 'l3num'") forgot to remove the attribute
present check when removing the related variable.

This commit removes that check to ensure consistency.

Signed-off-by: Lin Ma <linma@zju.edu.cn>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>