Fix CleanCommand not to throw FileNotFoundExceptions

Fix CleanCommand not to throw FileNotFoundExceptions

When CleanCommand is collecting the files and folders to be deleted
it may happen that the list of directories contains obsolete entries.
E.g. a folder and its parent folder may be in the list. Only the
parent folder would be sufficient.

This was a reason for hitting FileNotFoundExceptions when finally
trying to delete the files and folders. Improve CleanCommand
to ignore files to be deleted which are already gone.

Bug: 514434
Change-Id: I10caa01bfb9cec5967dfdaea50c6e4a713eeeabd
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>