migrator: Disable branch detection
The detection seems to be broken and produces invalid branches. Disable it altogether, and let people configure their branch later on. This is more reliable and we have nearly no people with a branch set in the router on the old updater anyway.
A problem from https://forum.turris.cz/t/migrace-updater-na-turrisu-hw-verze-1-x/2915/16.