Revert "If feature flag is enabled, replace the old login/registration pages with the new combined login/registration page."

This reverts commit f40447b3.

Conflicts:
	lms/urls.py

Revert "Add support for external auth on the combined login/registration page"

This reverts commit 98875339.

Conflicts:
	lms/templates/courseware/mktg_course_about.html
	lms/urls.py