Well I found a folder in
/opt from which I don't know the origin.
When I do
ls -la /opt it gives me
[email protected]:~$ ls -la /opt/ insgesamt 16 drwxr-xr-x 4 root root 4096 Sep 12 23:25 . drwxr-xr-x 23 root root 4096 Sep 9 15:44 .. drwxr-xr-x 2 clickpkg clickpkg 4096 Aug 13 13:45 click.ubuntu.com drwxr-xr-x 3 root root 4096 Jul 9 21:49 google
It is software you installed yourself (/opt/ is purely used for 3rd party applications you install yourself from mostly pre-compiled archives (tar.gz or something similar) and not from USC or a debian install file (if software did automatically get installed there from Ubuntu Software Center or even a debian installer (.DEB). The FileSystem Hierarchy we use has a strict rule on using /opt/)).
/opt/ is not used by "apt-get". If you want to get rid of it all you need to do to is
sudo rm -rf /opt/clickpkg. All software related to it -must- be in that directory when using /opt/ as an installation location.