[SPARK-6285][SQL]Remove ParquetTestData in SparkBuild.scala and in README.md

[OopsOutOfMemory]

This is a following clean up PR for #5010
This will resolve issues when launching hive/console like below:

<console>:20: error: object ParquetTestData is not a member of package org.apache.spark.sql.parquet
       import org.apache.spark.sql.parquet.ParquetTestData

[AmplabJenkins]

Can one of the admins verify this patch?

[liancheng]

ok to test

[liancheng]

This LGTM. Thanks for fixing this! I can merge it after it passes Jenkins

[OopsOutOfMemory]

ok, thanks @liancheng

[SparkQA]

Test build #28624 has started for PR 5032 at commit 2996aeb.

• This patch merges cleanly.

[SparkQA]

Test build #28624 has finished for PR 5032 at commit 2996aeb.

• 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/28624/
Test PASSed.