[SPARK-17525][PYTHON] Remove SparkContext.clearFiles() from the PySpark API as it was removed from the Scala API prior to Spark 2.0.0
[SPARK-17525][PYTHON] Remove SparkContext.clearFiles() from the PySpark API as it was removed from the Scala API prior to Spark 2.0.0

## What changes were proposed in this pull request?

This pull request removes the SparkContext.clearFiles() method from the PySpark API as the method was removed from the Scala API in 8ce645d4eeda203cf5e100c4bdba2d71edd44e6a. Using that method in PySpark leads to an exception as PySpark tries to call the non-existent method on the JVM side.

## How was this patch tested?

Existing tests (though none of them tested this particular method).

Author: Sami Jaktholm <sjakthol@outlook.com>

Closes #15081 from sjakthol/pyspark-sc-clearfiles.

(cherry picked from commit b5bfcddbfbc2e79d3d0fbd43942716946e6c4ba3)
Signed-off-by: Sean Owen <291c18f3fb7528c712d9098b0e50a515ea0b91d5@cloudera.com>

gioenn/xSpark,gioenn/xSpark,gioenn/xSpark,gioenn/xSpark,gioenn/xSpark,gioenn/xSpark,gioenn/xSpark