apparmor: Simplify obtain the newest label on a cred

In aa_get_task_label(), aa_get_newest_cred_label(__task_cred(task))
can do the same things as aa_get_newest_label(__aa_task_raw_label(task)),
so we can replace it and remove __aa_task_raw_label() to simplify the code.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>