netfilter: xt_CT: Remove redundant assignment to ret

Variable 'ret' is set to zero but this value is never read as it is
overwritten with a new value later on, hence it is a redundant
assignment and can be removed

Clean up the following clang-analyzer warning:

net/netfilter/xt_CT.c:175:2: warning: Value stored to 'ret' is never read [clang-analyzer-deadcode.DeadStores]

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>