remove legacy student dashboard

[deborahgu]

There's a student dashboard in the student djangoapp. However, that dashboard is no longer reachable. The primary view in views/dashboard.py has an early short-circuit if learner home MFE is enabled.

    if learner_home_mfe_enabled():
        return redirect(settings.LEARNER_HOME_MICROFRONTEND_URL)

Learner Home MFE is always enabled, now, because the legacy behavior was deprecated and removed. This page was simply missed during that removal.

acceptance criteria

• Remove dashboard.py
• redirect the URL to learner dashboard MFE, which already redirects to authn if not logged in.
• verify all functionality.