Well, my docs were not very good. So I am sure you already have removed it via
pkg
. The only other thing I think you may need to remove are the directories it created which are listed here:
Synth, plus if the repository was self hosted, this file probably needs to be changed to "yes":
/usr/local/etc/pkg/repos/FreeBSD.conf
I can't remember if synth created
/usr/local/etc/pkg/repos/FreeBSD.conf or if I had to. I had my synth instance on another machine and used that machine as a repo so my set up was different, but it was also a few months ago and I didn't keep docs around because I stopped using synth.
Hope that was at least somewhat helpful, if not a bit of a ramble