[SPARK-14969][MLLIB] Remove duplicate implementation of compute in LogisticGradient

## What changes were proposed in this pull request?

This PR removes duplicate implementation of compute in LogisticGradient class

## How was this patch tested?

unit tests

Author: dding3 <dingding@dingding-ubuntu.sh.intel.com>

Closes #12747 from dding3/master.

mllib/src/main/scala/org/apache/spark/ml/ann/Layer.scala

@@ -577,13 +577,6 @@ private[ann] object FeedForwardModel {
  * @param dataStacker data stacker
  */
 private[ann] class ANNGradient(topology: Topology, dataStacker: DataStacker) extends Gradient {
-
-  override def compute(data: Vector, label: Double, weights: Vector): (Vector, Double) = {
-    val gradient = Vectors.zeros(weights.size)
-    val loss = compute(data, label, weights, gradient)
-    (gradient, loss)
-  }
-
   override def compute(
     data: Vector,
     label: Double,

mllib/src/main/scala/org/apache/spark/mllib/optimization/Gradient.scala

@@ -146,12 +146,6 @@ class LogisticGradient(numClasses: Int) extends Gradient {
 
   def this() = this(2)
 
-  override def compute(data: Vector, label: Double, weights: Vector): (Vector, Double) = {
-    val gradient = Vectors.zeros(weights.size)
-    val loss = compute(data, label, weights, gradient)
-    (gradient, loss)
-  }
-
   override def compute(
       data: Vector,
       label: Double,