Remove Zendesk component from Learner Dashboard

[jsnwesson]

The Learner Dashboard is no longer using the Zendesk component. The original users was an internal team from 2U, which have since moved the implementation into a private repo via frontend plugin.

Acceptance Criteria:

• Remove ZendeskFab component.
• Update App.test.jsx to remove mock
• Remove dependency react-zendesk
• Update Jest snapshot
• Link PR to issue ticket ZendeskFab · Issue #225 · openedx/frontend-app-learner-dashboard and close ticket
• Link PR to DEPR ticket and close ticket