[SPARK-5543][WebUI] Remove unused import JsonUtil from from JsonProtocol

Simple PR to Remove unused import JsonUtil from from org.apache.spark.util.JsonProtocol.scala which fails builds with older versions of hadoop-core
This import is unused. It was introduced in PR #4029 https://github.com/apache/spark/pull/4029 as a part of JIRA SPARK-5231

Author: nemccarthy <nathan@nemccarthy.me>

Closes #4320 from nemccarthy/master and squashes the following commits:

8e34a11 [nemccarthy] [SPARK-5543][WebUI] Remove unused import JsonUtil from from org.apache.spark.util.JsonProtocol.scala which fails builds with older versions of hadoop-core