refactor!: remove no longer used addGlobalThemeStyles helper (#10673) (#10676)

Co-authored-by: Serhii Kulykov <iamkulykov@gmail.com>

Author: vaadin-bot

packages/vaadin-themable-mixin/register-styles.d.ts

@@ -1,9 +1,6 @@
-import type { CSSResultGroup } from 'lit';
-
 /**
  * @license
  * Copyright (c) 2017 - 2025 Vaadin Ltd.
  * This program is available under Apache License Version 2.0, available at https://vaadin.com/license/
  */
 export { registerStyles, css, unsafeCSS } from './vaadin-themable-mixin.js';
-export const addGlobalThemeStyles: (id: string, ...styles: CSSResultGroup[]) => void;

packages/vaadin-themable-mixin/register-styles.js

@@ -4,20 +4,3 @@
  * This program is available under Apache License Version 2.0, available at https://vaadin.com/license/
  */
 export { registerStyles, css, unsafeCSS } from './vaadin-themable-mixin.js';
-
-/**
- * This is for use internally by Lumo and Material styles.
- *
- * @param {string} id the id to set on the created element, only for informational purposes
- * @param  {CSSResultGroup[]} styles the styles to add
- */
-export const addGlobalThemeStyles = (id, ...styles) => {
-  const styleTag = document.createElement('style');
-  styleTag.id = id;
-  styleTag.textContent = styles
-    .map((style) => style.toString())
-    .join('\n')
-    .replace(':host', 'html');
-
-  document.head.insertAdjacentElement('afterbegin', styleTag);
-};