hmm, having a second look into message-disassociate-draft, I am 
still not sure what is supposed to happen there.

anyway, my motivation for the patch was to eliminate a rather 
annoying (since unnecessary) question to remove a non-existing 
backup/auto save file. here is the minimal patch (without any 
further changes to the code) doing so.

I think, it makes sense to also check for the existing file-name 
to only remove a backup file if there is a properly saved message, 
too.