Remove an inaccurate javadoc reference, which referred to an artifact…

… from an earlier draft version of the Extensions API.

This is google#322 by @lucastsa.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=119896497

value/src/main/java/com/google/auto/value/extension/AutoValueExtension.java

@@ -114,8 +114,7 @@ public Set<String> consumeProperties(Context context) {
 
   /**
    * Generates the source code of the class named {@code className} to extend
-   * {@code classToExtend}, with the original annotated class of
-   * {@code classToImplement}.  The generated class should be final if {@code isFinal}
+   * {@code classToExtend}. The generated class should be final if {@code isFinal}
    * is true; otherwise it should be abstract.
    *
    * @param context The {@link Context} of the code generation for this class.