[MINOR][MLLIB] Remove TODO comment DecisionTreeModel.scala

## What changes were proposed in this pull request?

This PR fixes the following line and the related code. Historically, this code was added in [SPARK-5597](https://issues.apache.org/jira/browse/SPARK-5597). After [SPARK-5597](https://issues.apache.org/jira/browse/SPARK-5597) was committed, [SPARK-3365](https://issues.apache.org/jira/browse/SPARK-3365) is fixed now. Now, we had better remove the comment without changing persistent code.

```scala
-        categories: Seq[Double]) { // TODO: Change to List once SPARK-3365 is fixed
+        categories: Seq[Double]) {
```

## How was this patch tested?

Pass the Jenkins tests.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #11966 from dongjoon-hyun/change_categories_type.

mllib/src/main/scala/org/apache/spark/mllib/tree/model/DecisionTreeModel.scala

@@ -156,7 +156,7 @@ object DecisionTreeModel extends Loader[DecisionTreeModel] with Logging {
         feature: Int,
         threshold: Double,
         featureType: Int,
-        categories: Seq[Double]) { // TODO: Change to List once SPARK-3365 is fixed
+        categories: Seq[Double]) {
       def toSplit: Split = {
         new Split(feature, threshold, FeatureType(featureType), categories.toList)
       }