Removing refrence to debian_ubuntu.sh installation script

Created by: francisoud

I tried this installation script (https://raw.github.com/gitlabhq/gitlabhq/master/doc/debian_ubuntu.sh) but it made more harm than good. ruby installation failed but there was so many log I didn't notice, then latest steps failed because some things were missing (I will update them in next commits) etc.

The main problem is that it brings confusion, there are too many installation script (an therefore 'ways to install') on this wiki (cleaning up)