Remove ntpd, reload apparmor

"openntpd" replaces "ntp" on Ubuntu.

dpkg will fail on post-install of openntpd with:
  Starting openntpd: /etc/openntpd/ntpd.conf: Permission denied

if the ntp apparmor profile is still applied.
Even after the ntp package purge, it's still in the cache.
Forget ntpd apparmor profile

According to https://bugs.launchpad.net/ubuntu/+source/openntpd/+bug/782116/comments/10 ,
this is a better way of handling the issue.
Merge pull request #1 from klamontagne/master

Remove ntpd, reload apparmor