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

[nemccarthy]

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 #4029 as a part of JIRA SPARK-5231

[AmplabJenkins]

Can one of the admins verify this patch?

[JoshRosen]

Jenkins, this is ok to test.

[SparkQA]

Test build #26573 has started for PR 4320 at commit 8e34a11.

• This patch merges cleanly.

[SparkQA]

Test build #26573 has finished for PR 4320 at commit 8e34a11.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/26573/
Test PASSed.

[JoshRosen]

Thanks for catching and fixing this. LGTM, so I'll merge this into master (1.3.0).