Revert enable login/registration

With regret.  @stephensanchez @rlucioni please review, especially the resolution of conflicts with email opt-in.

Revert "If feature flag is enabled, replace the old login/registration pages with the new combined login/registration page."

This reverts commit f40447b3c8f3946c2edbd21c92fe743e9f54a39d.

Conflicts:
	lms/urls.py

Revert "Add support for external auth on the combined login/registration page"

This reverts commit 988753395fa7509472b99b420057e151ce213399.

Conflicts:
	lms/templates/courseware/mktg_course_about.html
	lms/urls.py
Nitpick: two spaces is the standard indentation size in this file. Could you change this from four to two spaces?