I tested, the hiding of the "Other..." option in the "Download from:" drop down box, by editing Ubuntu.info and removing the line "MirrorsFile-amd64: Ubuntu.mirrors". With the chnages, when I ran software-properties-gtk I no longer saw an "Other..." option and so the choose download server dialog never appeared.
I tested, the hiding of the "Other..." option in the "Download from:" drop down box, by editing Ubuntu.info and removing the line "MirrorsFile-amd64: Ubuntu.mirrors". With the chnages, when I ran software- properties- gtk I no longer saw an "Other..." option and so the choose download server dialog never appeared.