Mhh.. in fact if you are in the "I'm not sure I want it forever" area,
you probably should better import it in a new DB, yes.
Noted.
To remove it from the DB you have to use the scary command:
mtn db kill_branch_certs_locally BRANCH-NAME
I'll run any scary command you like (on a branch I'm not developing myself), so long as you don't want me to do a sync afterwards...
In order to get rid of the extra file size you'd have to create a new
DB and locally sync the branches you're interested in into the new DB,
and then drop the old one.
Eeek. That isn't good. I would have thought that this was a pretty likely use case, if you compare with how CVS or Subversion is used.