refactor(submission): 删掉提交接口里走不到的验证码分支

captcha 在 CreateSubmissionSerializer 里是 required=False，视图里只有
data.get("captcha") 为真才校验，而前端只在注册表单发验证码，提交代码这
条路径从来不传，分支永远进不去。

验证码现在只服务注册流程。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

submission/serializers.py

@@ -39,7 +39,6 @@ class CreateSubmissionSerializer(serializers.Serializer):
     code = serializers.CharField(max_length=1024 * 1024)
     contest_id = serializers.IntegerField(required=False)
     problemset_id = serializers.IntegerField(required=False)
-    captcha = serializers.CharField(required=False)
 
 
 class ShareSubmissionSerializer(serializers.Serializer):