Align mitx-theme unauthenticated nav templates with upstream marketing-link changes

[Copilot]

What are the relevant tickets?

• hq#12099

What's this PR do?

Removes the deprecated ENABLE_MKTG_SITE gating from the MITx unauthenticated navbar overrides so marketing links render unconditionally, matching openedx-platform#38720 while preserving MITx-specific template behavior.

• Header navbar

  • Removes mktg_site_enabled from lms/templates/header/navbar-not-authenticated.html
  • Drops the now-unused reverse and text_type imports
  • Keeps COURSES_ARE_BROWSABLE around the Courses link

• Legacy nav variants

  • Removes the ENABLE_MKTG_SITE conditional from:
    • lms/templates/navigation/navbar-not-authenticated.html
    • lms/templates/navigation/bootstrap/navbar-not-authenticated.html
  • Leaves MITx-specific content intact
  • Keeps the upstream indentation/style shape in the non-bootstrap nav template

Example of the effective change:

- % if static.get_value('ENABLE_MKTG_SITE', settings.FEATURES.get('ENABLE_MKTG_SITE', False)):
    <a href="${marketing_link('HOW_IT_WORKS')}">${_("How it Works")}</a>
    % if settings.FEATURES.get('COURSES_ARE_BROWSABLE'):
      <a href="${marketing_link('COURSES')}">${_("Courses")}</a>
    % endif
    <a href="${marketing_link('SCHOOLS')}">${_("Schools")}</a>
- % endif

How should this be manually tested?

• Load the LMS in a logged-out state and verify the unauthenticated nav shows:
  • How it Works
  • Courses (only when COURSES_ARE_BROWSABLE is enabled)
  • Schools
• Check both navbar variants render correctly without relying on ENABLE_MKTG_SITE

Where should the reviewer start?

• lms/templates/header/navbar-not-authenticated.html
• lms/templates/navigation/navbar-not-authenticated.html

Any background context you want to provide?

This repo does not contain the optional upstream-affected overrides for:

• themes/stanford-style/lms/templates/footer.html
• lms/templates/instructor/edx_ace/addbetatester/email/body.html
• lms/templates/instructor/edx_ace/allowedenroll/email/body.html

Screenshots (if appropriate)

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What GIF best describes this PR or how it makes you feel?

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