fix: remove dead promise polyfill import from polyfillPromise.js

[stareezy-1]

Summary

Removes the dead else branch in polyfillPromise.js that imports the promise package. Since Hermes is the only supported JS engine and always provides a native Promise, hasPromise() is always true and the polyfill path can never execute.

Fixes #57702

Changelog:

[GENERAL] [REMOVED] - Dead promise polyfill code path in polyfillPromise.js

Problem

polyfillPromise.js conditionally imports the promise package (~15KB) via ../Promise in an else branch that can never execute:

if (global?.HermesInternal?.hasPromise?.()) {
  // Always true — Hermes is the only supported engine
} else {
  // Dead code — promise package is bundled but never used
  polyfillGlobal('Promise', () => require('../Promise').default);
}

This adds unnecessary weight to every React Native app's JS bundle.

Changes

• Removed the dead else branch
• Removed the unused polyfillGlobal import
• Simplified the condition since Hermes is guaranteed
• Preserved the __DEV__ promise rejection tracking setup

Test Plan

• The promise package is no longer bundled in the JS output
• Promise behavior is unchanged (Hermes native Promise was always used in practice)
• Dev-mode rejection tracking still works via HermesInternal.enablePromiseRejectionTracker

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