[SPARK-10315] remove document on spark.akka.failure-detector.threshold

https://issues.apache.org/jira/browse/SPARK-10315

this parameter is not used any longer and there is some mistake in the current document , should be 'akka.remote.watch-failure-detector.threshold'

Author: CodingCat <zhunansjtu@gmail.com>

Closes #8483 from CodingCat/SPARK_10315.

docs/configuration.md

@@ -906,16 +906,6 @@ Apart from these, the following properties are also available, and may be useful
 #### Networking
 <table class="table">
 <tr><th>Property Name</th><th>Default</th><th>Meaning</th></tr>
-<tr>
-  <td><code>spark.akka.failure-detector.threshold</code></td>
-  <td>300.0</td>
-  <td>
-     This is set to a larger value to disable failure detector that comes inbuilt akka. It can be
-     enabled again, if you plan to use this feature (Not recommended). This maps to akka's
-     `akka.remote.transport-failure-detector.threshold`. Tune this in combination of
-     `spark.akka.heartbeat.pauses` and `spark.akka.heartbeat.interval` if you need to.
-  </td>
-</tr>
 <tr>
   <td><code>spark.akka.frameSize</code></td>
   <td>128</td>