cifs: remove unused variable 'sid_user'

fs/cifs/cifsacl.c:43:30: warning:
 sid_user defined but not used [-Wunused-const-variable=]

It is never used, so remove it.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Steve French <stfrench@microsoft.com>