refactor: remove no-op legacy.proxySsrExternalModules (#20013)

Author: sapphi-red

packages/vite/src/node/config.ts

@@ -513,17 +513,6 @@ export interface ExperimentalOptions {
 }
 
 export interface LegacyOptions {
- /**
- * In Vite 4, SSR-externalized modules (modules not bundled and loaded by Node.js at runtime)
- * are implicitly proxied in dev to automatically handle `default` and `__esModule` access.
- * However, this does not correctly reflect how it works in the Node.js runtime, causing
- * inconsistencies between dev and prod.
- *
- * In Vite 5, the proxy is removed so dev and prod are consistent, but if you still require
- * the old behaviour, you can enable this option. If so, please leave your feedback at
- * https://github.com/vitejs/vite/discussions/14697.
- */
- proxySsrExternalModules?: boolean
  /**
  * In Vite 6.0.8 and below, WebSocket server was able to connect from any web pages. However,
  * that could be exploited by a malicious web page.
@@ -719,7 +708,6 @@ export const configDefaults = Object.freeze({
  removeSsrLoadModule: undefined,
  },
  legacy: {
- proxySsrExternalModules: false,
  skipWebSocketTokenCheck: false,
  },
  logLevel: 'info',