netfilter: nat: remove unnecessary 'else if' branch

Since a pseudo-random starting point is used in finding a port in
the default case, that 'else if' branch above is no longer a necessity.
So remove it to simplify code.

Signed-off-by: Xiaozhou Liu <liuxiaozhou@bytedance.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>