apparmor: cleanup unused functions in file.h

After changes in commit 33bf60ca ("LSM: Infrastructure management of
the file security"), aa_alloc_file_ctx() and aa_free_file_ctx() are no
longer used, so remove them, and also remove aa_get_file_label() because
it seems that it's never been used before.

Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>