[SPARK-6285] [SQL] Removes unused ParquetTestData and duplicated TestGroupWriteSupport

All the contents in this file are not referenced anywhere and should have been removed in #4116 when I tried to get rid of the old Parquet test suites.

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Author: Cheng Lian <[email protected]>

Closes #5010 from liancheng/spark-6285 and squashes the following commits:

06ed057 [Cheng Lian] Removes unused ParquetTestData and duplicated TestGroupWriteSupport

Author: liancheng