ci: remove downstream unmanaged dependency check (#14663)

The repositories checked by this workflow (`googleapis/java-bigtable`,
`java-firestore` and `java-pubsub`) are archived. Their libraries now
live in this repository and are already covered by
`unmanaged_dependency_check.yaml`, which runs the check against
`gapic-libraries-bom`.

This also unblocks #14661, where the workflow is failing.

Author: blakeli0