Fix media.user_id migration down(): Remove foreign key before user_id column

Summary: Fix media.user_id migration down(): Remove foreign key before user_id column

Test Plan:
- Without the fix: run `./minion migrations:run --group=3-0 --down` and see an error
- With the fix: run `./minion migrations:run --group=3-0 --up` and see all downgrades run fine

Reviewers: shadowhand

Reviewed By: shadowhand

Differential Revision: https://phabricator.ushahidi.com/D67