[SPARK-3138][SQL] sqlContext.parquetFile should be able to take a single file as parameter

```if (!fs.getFileStatus(path).isDir) throw Exception``` make no sense after this commit #1370

be careful if someone is working on SPARK-2551, make sure the new change passes test case ```test("Read a parquet file instead of a directory")```

Author: chutium <teng.qiu@gmail.com>

Closes #2044 from chutium/parquet-singlefile and squashes the following commits:

4ae477f [chutium] [SPARK-3138][SQL] sqlContext.parquetFile should be able to take a single file as parameter

(cherry picked from commit 48f42781dedecd38ddcb2dcf67dead92bb4318f5)
Signed-off-by: Michael Armbrust <michael@databricks.com>