Remove deprecation of DBCursor#explain

Since we ended up adding explain support to FindIterable,
no reason to leave it deprecated in the legacy API.

JAVA-5154

Author: jyemin

driver-legacy/src/main/com/mongodb/DBCursor.java

@@ -369,7 +369,6 @@ public DBCursor maxTime(final long maxTime, final TimeUnit timeUnit) {
      * @mongodb.driver.manual reference/command/explain Explain Output
      * @mongodb.server.release 3.0
      */
-    @Deprecated
     public DBObject explain() {
         return executor.execute(getQueryOperation(collection.getObjectCodec())
                         .asExplainableOperation(null, getDefaultCodecRegistry().get(DBObject.class)),