remove docs

Author: Davies Liu

docs/configuration.md

@@ -630,15 +630,6 @@ Apart from these, the following properties are also available, and may be useful
     output directories. We recommend that users do not disable this except if trying to achieve compatibility with
     previous versions of Spark. Simply use Hadoop's FileSystem API to delete output directories by hand.</td>
 </tr>
-<tr>
-    <td><code>spark.hadoop.cloneConf</code></td>
-    <td>false</td>
-    <td>If set to true, clones a new Hadoop <code>Configuration</code> object for each task.  This
-    option should be enabled to work around <code>Configuration</code> thread-safety issues (see
-    <a href="https://issues.apache.org/jira/browse/SPARK-2546">SPARK-2546</a> for more details).
-    This is disabled by default in order to avoid unexpected performance regressions for jobs that
-    are not affected by these issues.</td>
-</tr>
 <tr>
     <td><code>spark.executor.heartbeatInterval</code></td>
     <td>10000</td>