Indeed, you choose a mysql install without installing redmine-mysql, so that broke your redmine install.
However, you have a point here : one should be able to remove the package anyway. I'll fix this in the 0.9.1 release, which should be backported, in coming weeks or so. Let me know if you need help removing the package (or just comment the "set -e" line at the top of /var/lib/dpkg/info/redmine.postinst)
signature.asc
Description: OpenPGP digital signature
