fix: duplicate import dropdown for export page

[KristinAoki]

Description

This PR removes the unused variable definition for studio_home_mfe_url and fixes the duplicate import options in the Tools dropdown when the waffle flag for the export mfe page is enabled.

Before

After

Supporting information

JIRA Ticket: TNL-11052

When I view from legacy page, the tools dropdown shows import twice but no export
However, from any new course auth page, the dropdown includes import and export

Testing instructions

1. Enable contentstore.new_studio_mfe.use_new_export_page in django admin.
2. Open a course in studio.
3. Click on the tool dropdown.
4. Should have Import, Export, Checklist

Deadline

None

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.