30 March 2010 at 12:50am
(Last edited: 30 March 2010 12:50am),
You seem to have upgraded at a previous moment. Simply remove the column by hand (or with your preferred SQL application). Columns named _obsolete can generally be removed without problems as they are remnants of old code.
(Not handling existing _obsolete columns could/should be considered a bug in /dev/build)