[Bug fix][Refactor] Course scores & Download

- Fix a bug that scores in course mode cannot calculate rating
- Make some optimization for downloading songs

latest version/server/course.py

@@ -1,6 +1,5 @@
 from core.constant import Constant
 from core.course import UserCourseList
-from core.error import ArcError
 from core.item import ItemCore
 from core.sql import Connect
 from core.user import UserOnline

latest version/server/others.py

@@ -35,8 +35,6 @@ def download_song(user_id):
         x.url_flag = json.loads(request.args.get('url', 'true'))
         if x.url_flag and x.is_limited:
             raise RateLimit('You have reached the download limit.', 903)
-        if x.url_flag:
-            x.clear_download_token()
 
         x.add_songs()
         return success_return(x.urls)