As an overall clean-up effort to free space on our mirror volumes,
stop mirroring source packages for the ubuntu package repository.
According to codesearch, no jobs in our system add deb-src lines for
it so this should be entirely safe. It also won't take effect
immediately, and will require a manual step to prune the old
packages from the filesystem once this configuration update has
deployed.
Change-Id: Ie2aa6bdf43721c9ceacea54f7f5bc928883287a7
1474d10652