[SPARK-6037][SQL] Avoiding duplicate Parquet schema merging

`FilteringParquetRowInputFormat` manually merges Parquet schemas before computing splits. However, it is duplicate because the schemas are already merged in `ParquetRelation2`. We don't need to re-merge them at `InputFormat`.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #4786 from viirya/dup_parquet_schemas_merge and squashes the following commits:

ef78a5a [Liang-Chi Hsieh] Avoiding duplicate Parquet schema merging.