If "manage.py buildlist" is ran, it doesn't list the failed builds. Thus there doesn't appear to be a way to delete the failed builds from the database since there is no id to pass to "manage.py builddelete".
Submitted for review http://git.yoctoproject.org/cgit/cgit.cgi/poky-contrib/log/?h=adamian/20150603_remove_managed
Verified on master: bc2a55547f5ee1d15e6bf3b201582bdfb661c534