Fixed issue #1718. Removed reference to non-existent js file.

**Issue**: #1718

**Problem**: "Go to the Submissions view of an assignment; we're requesting a missing `submissions_manager.js`."

**Fix**: I removed the file as, currently, the only place that file is referenced in the codebase is in the `javascript_include_tag` for the submissions page. The js file was removed in commit named "Clean up unnecessary stuff", 4ace620fbd4f03cb291b69c710065c3818d34b44, where a lot of partials were also removed.

**Testing**: both tests still work. This is expected as just a missing reference was removed from a client-side file.