add-apt-repository does not remove URI if dist is different from local dist

[Dan Streetman]

if a configured repository has a 'dist' value different from the local system's codename, add-apt-repository will not remove it.

For example, if this line is currently configured:

deb [arch=amd64] http://dl.google.com/linux/chrome/deb/ stable main

It will not be removed with the command:

sudo add-apt-repository -r -U http://dl.google.com/linux/chrome/deb/