netfilter: conntrack: Remove useless assignment statements

The old_size assignment here will not be used anymore

The clang_analyzer complains as follows:

Value stored to 'old_size' is never read

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: luo penghao <luo.penghao@zte.com.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>