DATAMONGO-2040 - Polishing.

Deprecate Index.Duplicates. Remove unused imports. Mention deprecation in What's new.

Original pull request: #599.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/index/Index.java

@@ -36,6 +36,10 @@ import org.springframework.util.StringUtils;
 @SuppressWarnings("deprecation")
 public class Index implements IndexDefinition {
 
+	/**
+	 * @deprecated since 2.1. No longer supported by MongoDB as of server version 3.0.
+	 */
+	@Deprecated
 	public enum Duplicates {
 		RETAIN
 	}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/query/IndexUnitTests.java

@@ -24,7 +24,6 @@ import org.springframework.data.domain.Sort.Direction;
 import org.springframework.data.mongodb.core.index.GeoSpatialIndexType;
 import org.springframework.data.mongodb.core.index.GeospatialIndex;
 import org.springframework.data.mongodb.core.index.Index;
-import org.springframework.data.mongodb.core.index.Index.Duplicates;
 
 /**
  * Unit tests for {@link Index}.

src/main/asciidoc/new-features.adoc

@@ -15,6 +15,7 @@
 * <<mongo.transactions, MongoDB 4.0 Transaction>> support and a MongoDB-specific transaction manager implementation.
 * <<mongodb.repositories.queries.sort,Default sort specifications for repository query methods>> using `@Query(sort=…)`.
 * <<mongo-template.find-and-replace,findAndReplace>> support through imperative and reactive Template APIs.
+* Deprecation of `dropDups` in `@Indexed` and `@CompoundIndex` as MongoDB server 3.0 and newer do not support `dropDups` anymore.
 
 [[new-features.2-0-0]]
 == What's New in Spring Data MongoDB 2.0