Clean up unused functions from SSR (#20995)

Author: sebmarkbage

packages/react-dom/src/server/DOMMarkupOperations.js

@@ -8,7 +8,6 @@
  */
 
 import {
-  ID_ATTRIBUTE_NAME,
   ROOT_ATTRIBUTE_NAME,
   BOOLEAN,
   OVERLOADED_BOOLEAN,
@@ -24,16 +23,6 @@ import quoteAttributeValueForBrowser from './quoteAttributeValueForBrowser';
  * Operations for dealing with DOM properties.
  */
 
-/**
- * Creates markup for the ID property.
- *
- * @param {string} id Unescaped ID.
- * @return {string} Markup string.
- */
-export function createMarkupForID(id: string): string {
-  return ID_ATTRIBUTE_NAME + '=' + quoteAttributeValueForBrowser(id);
-}
-
 export function createMarkupForRoot(): string {
   return ROOT_ATTRIBUTE_NAME + '=""';
 }

packages/react-dom/src/shared/DOMProperty.js

@@ -62,7 +62,6 @@ export const ATTRIBUTE_NAME_START_CHAR =
 export const ATTRIBUTE_NAME_CHAR =
   ATTRIBUTE_NAME_START_CHAR + '\\-.0-9\\u00B7\\u0300-\\u036F\\u203F-\\u2040';
 
-export const ID_ATTRIBUTE_NAME = 'data-reactid';
 export const ROOT_ATTRIBUTE_NAME = 'data-reactroot';
 export const VALID_ATTRIBUTE_NAME_REGEX = new RegExp(
   '^[' + ATTRIBUTE_NAME_START_CHAR + '][' + ATTRIBUTE_NAME_CHAR + ']*$',