[MLlib]remove unused import

minor thing. Let me know if jira is required.

Author: Yuhao Yang <hhbyyh@gmail.com>

Closes #5207 from hhbyyh/adjustImport and squashes the following commits:

2240121 [Yuhao Yang] remove unused import

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

@@ -19,7 +19,7 @@ package org.apache.spark.mllib.clustering
 
 import java.util.Random
 
-import breeze.linalg.{DenseVector => BDV, normalize, axpy => brzAxpy}
+import breeze.linalg.{DenseVector => BDV, normalize}
 
 import org.apache.spark.Logging
 import org.apache.spark.annotation.Experimental