apparmor: fix test error: WARNING in apparmor_unix_stream_connect

commit 88fec352 ("apparmor: make sure unix socket labeling is correctly updated.")
added the use of security_sk_alloc() which ensures the sk label is
initialized.

This means that the AA_BUG in apparmor_unix_stream_connect() is no
longer correct, because while the sk is still not being initialized
by going through post_create, it is now initialize in sk_alloc().
Remove the now invalid check.

Reported-by:  <syzbot+cd38ee04bcb3866b0c6d@syzkaller.appspotmail.com>
Fixes: 88fec352 ("apparmor: make sure unix socket labeling is correctly updated.")
Signed-off-by: John Johansen <john.johansen@canonical.com>