[flags] clean up enableUseEffectEventHook

[everettbu]

Mirror of facebook/react#35541
Original author: rickhanlonii

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This is landed everywhere

[greptile-apps]

Greptile Summary

This PR removes the enableUseEffectEventHook feature flag, which was set to true everywhere, making useEffectEvent a permanently enabled feature in React. The changes are consistent across all affected files:

• Removed flag definition from all feature flag files (main and platform-specific forks)
• Updated Dispatcher type to make useEffectEvent non-optional
• Removed conditional checks and directly added useEffectEvent to all dispatcher objects
• Removed @gate enableUseEffectEventHook annotations from tests
• Simplified code in ReactFiberCommitWork.js and ReactFiberFlags.js by removing nested conditionals

This is a standard feature flag cleanup with no functional changes since the flag was already true everywhere.

Confidence Score: 5/5

• Safe to merge - straightforward feature flag cleanup with no functional changes
• This is a mechanical refactoring that removes a feature flag that was already enabled everywhere. All changes follow the same pattern: remove the flag import/definition, remove conditional checks, and make the feature permanently available. The changes are consistent across all files and don't alter runtime behavior.
• No files require special attention

Important Files Changed

Filename	Overview
packages/shared/ReactFeatureFlags.js	Removed enableUseEffectEventHook flag definition (was set to true)
packages/react-reconciler/src/ReactInternalTypes.js	Changed useEffectEvent from optional to required in Dispatcher type, removed outdated TODO comment
packages/react-reconciler/src/ReactFiberCommitWork.js	Removed enableUseEffectEventHook import and flag check, now always processes event payloads
packages/react-reconciler/src/ReactFiberHooks.js	Removed enableUseEffectEventHook import and conditionals, added useEffectEvent directly to all dispatcher objects

[everettbu]

Upstream PR was closed or merged. Code is synced via branch mirror.