[SPARK-30124][MLLIB] unnecessary persist in PythonMLLibAPI.scala

### What changes were proposed in this pull request?
Removed unnecessary persist.

### Why are the changes needed?
Persist in `PythonMLLibAPI.scala` is unnecessary because later in `run()` of `gmmAlg` is caching the data.
https://github.com/apache/spark/blob/710ddab39e20f49e917311c3e27d142b5a2bcc71/mllib/src/main/scala/org/apache/spark/mllib/clustering/GaussianMixture.scala#L167-L171

### Does this PR introduce any user-facing change?
No

### How was this patch tested?
Manually

Closes apache#26758 from amanomer/improperPersist.

Authored-by: Aman Omer <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

Author: amanomer

mllib/src/main/scala/org/apache/spark/mllib/api/python/PythonMLLibAPI.scala

@@ -407,11 +407,7 @@ private[python] class PythonMLLibAPI extends Serializable {
 
     if (seed != null) gmmAlg.setSeed(seed)
 
-    try {
-      new GaussianMixtureModelWrapper(gmmAlg.run(data.rdd.persist(StorageLevel.MEMORY_AND_DISK)))
-    } finally {
-      data.rdd.unpersist()
-    }
+    new GaussianMixtureModelWrapper(gmmAlg.run(data.rdd))
   }
 
   /**

mllib/src/main/scala/org/apache/spark/mllib/clustering/GaussianMixture.scala

@@ -234,6 +234,7 @@ class GaussianMixture private (
       iter += 1
       compute.destroy()
     }
+    breezeData.unpersist()
 
     new GaussianMixtureModel(weights, gaussians)
   }