selinux: remove the wrong comment about multithreaded process handling

Since commit d9250dea ("SELinux: add boundary support and thread
context assignment"), SELinux has been supporting assigning per-thread
security context under a constraint and the comment was updated
accordingly. However, seems like commit d84f4f99 ("CRED: Inaugurate
COW credentials") accidentally brought the old comment back that doesn't
match what the code does.

Considering the ease of understanding the code, this patch just removes the
wrong comment.

Fixes: d84f4f99

 ("CRED: Inaugurate COW credentials")
Signed-off-by: Munehisa Kamata <kamatam@amazon.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>