Remove option "Pre-release updates (-proposed)" from the Updates tab for the devel release

[Jean-Baptiste Lallement]

software-properties-gtk 0.92.32, Ubuntu Trusty

software-properties-gtk offers the possibility to enable -proposed (in Updates/Install updates from cf screenshot)
While this options may make sense for stable releases (even though it is debatable) it is dangerous for people running the development release.

This item should be removed or hidden from the UI for the development release.

<https://wiki.ubuntu.com/SoftwareUpdates#settings>: "The 'When checking for updates, check for:' menu should contain options for 'All updates' (the default, -security + -updates + -backports), 'Security and recommended updates' (-security + -updates), and 'Security updates only' (-security). If your current update config does not match one of those three options (for example, if you have opted in to -proposed), there should be a fourth, checked, option: 'Custom', and this option should persist until you close System Settings. (UI for configuring -proposed should be provided by the Ubuntu Contributor Console.)"