This change is required to support the changes to the seed data repo
structure introduced in:
gitpython-developers/qa-assets#2

This moves most of the seed data related build steps into the OSS-Fuzz
Docker image build via `container-environment-bootstrap.sh`. This
includes moveing the dictionaries into that repo.

The fuzzing/README.md here should be updated in a follow-up with a link
to the qa-assets repo (and probably some context setting about corpora
in general) but I have opted to defer that as I think the functionality
added by the seed data improvements is valuable as is and shouldn't be
blocked by documentation writers block.