chore: Repalce message formatter mixin with useMessageFormatter [CW-3470] (#9986)

# Pull Request Template

## Description

Replaced the old messageFormatterMixin with a useMessageFormatter
composable

app/javascript/shared/mixins/specs/messageFormatterMixin.spec.js

@@ -1,17 +0,0 @@
-import { shallowMount } from '@vue/test-utils';
-import messageFormatterMixin from '../messageFormatterMixin';
-
-describe('messageFormatterMixin', () => {
-  it('returns correct plain text', () => {
-    const Component = {
-      render() {},
-      mixins: [messageFormatterMixin],
-    };
-    const wrapper = shallowMount(Component);
-    const message =
-      '<b>Chatwoot is an opensource tool. https://www.chatwoot.com</b>';
-    expect(wrapper.vm.getPlainText(message)).toMatch(
-      'Chatwoot is an opensource tool. https://www.chatwoot.com'
-    );
-  });
-});