selinux: fix build warning by removing the unused sid variable

This patch remove the unused variable sid
This fix the following build warning:
security/selinux/hooks.c:2921:6: warning: variable 'sid' set but not used [-Wunused-but-set-variable]

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>