Fix usr.bin.chromium-browser rm_conffile versioning (Closes: #1101071)
prior-version should be the first version to introduce the dpkg-maintscript invocation (here rm_conffile with the current version), not the one when the conffile should have initialy been removed.
See man 1 dpkg-maintscript-helper.
Fixes https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1101071
Note if the version this commit is shipped into is not 4.1.0~beta5-6, the rm_conffile will need to be changed.