Some context around this PR:

These proto files are no longer used in the main client library. They only exist to assist in migration for packages that need to support google-cloud-bigquery 2.x and 3.x. Instead, a dictionary of the REST resource is exposed for the BQML-related statistics. This choice was made for a few reasons:

• The backend team rarely publishes the updates they make to these protos to https://github.com/googleapis/googleapis/tree/master/google/cloud/bigquery Instead, historically the client libraries team has had to manually update them.
• Sometimes the BQML team makes (unintentional?) breaking changes to the proto definitions. Since these protos are only manually updated, those breaking changes are only caught much later.
• The proto files are only used by the Python client library, not any of the other client libraries.
• The proto files only include a small subset of the BigQuery v2 interface.
• The presence of these proto files occasionally confuses members of the backend team into thinking the BigQuery v2 API supports gRPC and/or GAPIC client library generation.

Resources

• [Breaking Change] Remove google.cloud.bigquery_v2 directory and related code generation
• chore!: remove google.cloud.bigquery_v2 code
• feat: add a static copy of legacy proto-based types

Towards #814