Cleanup after removing StreamableReader

Javadoc and unused imports!

core/src/main/java/org/elasticsearch/cluster/AbstractDiffable.java

@@ -22,7 +22,6 @@ package org.elasticsearch.cluster;
 import org.elasticsearch.common.Nullable;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
-import org.elasticsearch.common.io.stream.StreamableReader;
 
 import java.io.IOException;
 

core/src/main/java/org/elasticsearch/common/io/stream/Writeable.java

@@ -31,11 +31,6 @@ import java.io.IOException;
  *
  * Prefer implementing this interface over implementing {@link Streamable} where possible. Lots of code depends on {@linkplain Streamable}
  * so this isn't always possible.
- *
- * The fact that this interface extends {@link StreamableReader} should be consider vestigial. Instead of using its
- * {@link #readFrom(StreamInput)} method you should prefer using the Reader interface as a reference to a constructor that takes
- * {@link StreamInput}. The reasoning behind this is that most "good" readFrom implementations just delegated to such a constructor anyway
- * and they required an unsightly PROTOTYPE object.
  */
 public interface Writeable<T> { // TODO remove <T>
     /**
@@ -43,8 +38,12 @@ public interface Writeable<T> { // TODO remove <T>
      */
     void writeTo(StreamOutput out) throws IOException;
 
+    /**
+     * Read this object from a stream. Use a {@link Writeable.Reader} instead. This lives on for backwards compatibility but should be
+     * removed before 5.0.0GA. It is not deprecated because Diffable extends this interface and it shouldn't be deprecated there.
+     */
     default T readFrom(StreamInput in) throws IOException {
-        // See class javadoc for reasoning
+        // NORELEASE remove before 5.0.0GA
         throw new UnsupportedOperationException(
                 "Prefer calling a constructor or static method that takes a StreamInput to calling readFrom.");
     }

core/src/main/java/org/elasticsearch/common/xcontent/FromXContentBuilder.java

@@ -20,15 +20,11 @@
 package org.elasticsearch.common.xcontent;
 
 import org.elasticsearch.common.ParseFieldMatcher;
-import org.elasticsearch.common.io.stream.StreamableReader;
 
 import java.io.IOException;
 
 /**
  * Indicates that the class supports XContent deserialization.
- *
- * This interface is similar to what {@link StreamableReader} does, only it works with XContent serialization
- * instead of binary serialization.
  */
 public interface FromXContentBuilder<T> {
     /**

core/src/test/java/org/elasticsearch/cluster/serialization/DiffableTests.java

@@ -28,7 +28,6 @@ import org.elasticsearch.common.collect.ImmutableOpenMap;
 import org.elasticsearch.common.io.stream.BytesStreamOutput;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
-import org.elasticsearch.common.io.stream.StreamableReader;
 import org.elasticsearch.common.util.set.Sets;
 import org.elasticsearch.test.ESTestCase;