Remove wasm-test-streaming flag

Editor assessment

The subject states the removal exactly: the wasm-test-streaming flag is gone. The what is fully clear, but the why (obsolete after a refactor, or superseded by a newer harness) is unstated, and for a test-infrastructure removal the rationale is the interesting part. Type fix is defensible for removing a broken or dead flag; scope core is accurate. No body supplies the reasoning, and no footers appear beyond the PR number. Precise mechanically, silent motivationally.