net: sock: remove the unnecessary check in proto_register

tw_prot_cleanup will check the twsk_prot.

Fixes: 0f5907af

 ("net: Fix potential memory leak in proto_register()")
Cc: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>