Remove redundant assertion

It does look pretty dumb to be checking if something is `null` *after
calling an instance method on it*...

osu.Game/Scoring/Legacy/LegacyScoreDecoder.cs

@@ -187,8 +187,6 @@ internal static void PopulateMaximumStatistics(ScoreInfo score, WorkingBeatmap w
             var rulesetInstance = ruleset.CreateInstance();
             var scoreProcessor = rulesetInstance.CreateScoreProcessor();
 
-            Debug.Assert(rulesetInstance != null);
-
             // Populate the maximum statistics.
             HitResult maxBasicResult = rulesetInstance.GetHitResults()
                                                       .Select(h => h.result)