Remove deprecated observability_tracing feature flag

What does this MR do and why?

As a follow up to Add single observability feature flag and licen... (!158786 - merged) , we can now removed the deprecated FF as everything has been migrated to the new observability_features flag

Part of Part of Merge tracing/metrics/logging feature flags int... (gitlab-org/opstrace/opstrace#2847 - closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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How to set up and validate locally

• Enable observability_features FF
• Check that http://gdk.test:3443/flightjs/Flight/-/metrics http://gdk.test:3443/flightjs/Flight/-/logs and http://gdk.test:3443/flightjs/Flight/-/tracing are available