Drop support for GCS loading

[C0urante]

BigQuery's ingestion quotas have become increasingly permissive over time, and it's debatable whether the GCS batch loading mode for the connector is still useful. Additionally, supporting multiple write modes is a maintenance burden for the connector, and leaves the door open for divergences in behavior and bugs that affect multiple modes but are only noticed/fixed in one at a time.

If the GCS loading feature is no longer relevant for most users, we should consider removing it from the connector.