GCP: Avoid duplicate BigQuery namespace loads

[charliec05]

Summary

• delegate namespace existence validation to BigQueryMetastoreClient.removeParameters
• remove the catalog-level preflight load and empty-set short circuit
• verify that removing namespace properties performs exactly one client load

This makes removeProperties consistent with setProperties while preserving missing-namespace validation.

Closes #17059

Testing

• ./gradlew :iceberg-bigquery:test

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AI Disclosure

• Model: GPT-5
• Platform/Tool: OpenAI Codex
• Human Oversight: fully reviewed
• Prompt Summary: A detailed, repository-specific prompt requested removal of the redundant remote lookup, precise call-count coverage, formatting, and full BigQuery module validation.