chore(aisix): drop the retired observability.usage_event.request_headers

[jarvis9443]

api7/aisix#1299 retires observability.usage_event.request_headers. Which request headers an export records is now set per exporter, through the observability_exporters resource's own request_headers / upstream_response_headers. The key becomes a tombstone that still parses and only warns at boot. So this drops it from config: in values.yaml, from the CI values file, and from the README row.

It's a draft on purpose: it merges with the chart bump of the release that ships api7/aisix#1299. A 1.6.0 gateway still reads the key, so dropping it before then would remove a working setting for anyone on that image. No chart version/appVersion bump here.

🤖 Generated with Claude Code

Fixes api7/ticket-analysis#1051

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