Stop passing the now-obsolete mergebasebranch option

refs tails#12557 (closed), tails@3c9c4ca1

We can merge this clean-up MR once tails!381 (merged) has been merged into all our base branches, i.e. after 4.18 is out, assuming tails!381 (merged) is merged into stable before the 4.18 release.

Note to the reviewer: once this MR is merged, please notify developers so they revert tails@3c9c4ca1.