refactor: drop unreachable marketing-site branches from enrollment emails

With ENABLE_MKTG_SITE removed, get_email_params() now always sets
course_about_url via get_link_for_about_page(), which never returns None
(it falls back to the LMS course about URL). The instructor "allowed
enroll" and "add beta tester" ACE email templates branched on
`course_about_url is not None`, keeping an else-branch for the old
marketing-site-enabled case where course_about_url was None.

Those conditionals are now always true and their else-branches are dead:
- allowedenroll: "You can then enroll in {{ course_name }}."
- addbetatester: "Visit {{ site_name }} to enroll in this course and
  begin the beta test."

Collapse the always-true `course_about_url is not None` guards to the
unconditional branch and delete the unreachable text (both .txt and .html
variants). No behavior change for any reachable input.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>