chore(AI): paygInstanceStatsEvents flag cleanup (#10510)

This PR cleans up the paygInstanceStatsEvents flag. These changes were
automatically generated by AI and should be reviewed carefully.

Fixes #10507



🧹 AI Flag Cleanup Summary

This change removes the paygInstanceStatsEvents feature flag. The
changes
involve removing its definition from the codebase.

⚠️ No conditional logic for paygInstanceStatsEvents was found in the
provided
files. The logic may exist in another repository, and this change only
removes
the flag's definition.

🚮 Removed

• Configuration
• Removed paygInstanceStatsEvents flag definition from
src/lib/types/experimental.ts.
• Changelog
• Added an entry for the flag cleanup under a new [Unreleased] section
in
CHANGELOG.md.

📝 Why

The paygInstanceStatsEvents feature flag was marked as completed with
the
outcome "kept". This cleanup removes the flag's definition, following
our
standard process for retiring feature flags.

---------

Co-authored-by: unleash-bot <194219037+unleash-bot[bot]@users.noreply.github.com>
Co-authored-by: Thomas Heartman <thomas@getunleash.io>

src/lib/types/experimental.ts

@@ -57,7 +57,6 @@ export type IFlagKey =
     | 'customMetrics'
     | 'impactMetrics'
     | 'changeRequestApproverEmails'
-    | 'paygInstanceStatsEvents'
     | 'lifecycleGraphs'
     | 'addConfiguration'
     | 'filterFlagsToArchive'
@@ -266,10 +265,6 @@ const flags: IFlags = {
         process.env.UNLEASH_EXPERIMENTAL_IMPACT_METRICS,
         false,
     ),
-    paygInstanceStatsEvents: parseEnvVarBoolean(
-        process.env.UNLEASH_EXPERIMENTAL_PAYG_INSTANCE_STATS_EVENTS,
-        false,
-    ),
     lifecycleGraphs: parseEnvVarBoolean(
         process.env.UNLEASH_EXPERIMENTAL_LIFECYCLE_GRAPHS,
         false,