Removes println when removing unused *.class files.

#1525 removes unused *.class files created by the metabuid.
The implementation currently prints out a long list of *.class files
it’s about to remove. Unless something bad happens, this information is
not very useful to the user, so I suggest we don’t display it.
Merge pull request #1540 from sbt/wip/quieter-cleaning

Removes println when removing unused *.class files.