fix: Delete FileManipulation::getSonSubPath.

* src/dctc/java/com/dataiku/dctc/file/FileManipulation.java: Name was
  too confusing.
* src/dctc/test/com/dataiku/dctc/FileManipulationTest.java: Delete
  test.
* src/dctc/java/com/dataiku/dctc/copy/SyncComputer.java,
  src/dctc/java/com/dataiku/dctc/command/ListCommand.java: Adjust.