ci: remove downstream unmanaged dependency check

The workflow checks out googleapis/java-bigtable, googleapis/java-firestore
and googleapis/java-pubsub and runs the unmanaged dependency check against
their BOMs. All three repositories are archived since the libraries moved
into this repository, so the workflow only validates frozen copies of
their BOMs.

The libraries in this repository are already covered by
`unmanaged_dependency_check.yaml`, which runs the check built from the
pull request against the pull request's java-shared-dependencies and
`gapic-libraries-bom/pom.xml`. That BOM imports google-cloud-bigtable-bom,
google-cloud-firestore-bom and google-cloud-pubsub-bom, and the workflow
triggers on a superset of the paths of the removed workflow.

Author: blakeli0