[FLINK-3992][core] Do not implement deprecated Key interface

Author: reswqa

flink-core/src/main/java/org/apache/flink/types/DoubleValue.java

@@ -32,8 +32,7 @@
 public class DoubleValue
         implements Comparable<DoubleValue>,
                 ResettableValue<DoubleValue>,
-                CopyableValue<DoubleValue>,
-                Key<DoubleValue> {
+                CopyableValue<DoubleValue> {
     private static final long serialVersionUID = 1L;
 
     private double value;

flink-core/src/main/java/org/apache/flink/types/FloatValue.java

@@ -30,10 +30,7 @@
  */
 @Public
 public class FloatValue
-        implements Comparable<FloatValue>,
-                ResettableValue<FloatValue>,
-                CopyableValue<FloatValue>,
-                Key<FloatValue> {
+        implements Comparable<FloatValue>, ResettableValue<FloatValue>, CopyableValue<FloatValue> {
     private static final long serialVersionUID = 1L;
 
     private float value;

flink-core/src/main/java/org/apache/flink/types/NormalizableKey.java

@@ -34,7 +34,7 @@
  * normalized key length.
  */
 @Public
-public interface NormalizableKey<T> extends Comparable<T>, Key<T> {
+public interface NormalizableKey<T> extends Comparable<T> {
 
     /**
      * Gets the maximal length of normalized keys that the data type would produce to determine the