Offload hz-bbw6 from DockerLatent workloads

Changes

Move the bigtest builder to master-migration

• It has no real dependency on the output of amd64-ubuntu-2204.
• In fact, that dependency causes unnecessary delays.
• It should instead be a child of tarball-docker.

Remove hz-bbw6 from the valgrind DockerLatent pool

• The existing queues should handle this change without issue, but monitor them to confirm.
  valgrind still has 2 DockerLatent workers in its pool after this change.

bm-bbw1-docker removal

• faulty hardware, was removed from infra a couple weeks ago