Use Guava's `MoreFiles` for recursive deletion.

The Apache `FileUtils.deleteDirectory` has historically been unstable.
Most recently,
https://issues.apache.org/jira/projects/IO/issues/IO-862
causes `SymlinkValidatorTest` to fail at HEAD.

Change-Id: Id783a2b9fdd67608d72bdd3b6fbf03ccd4cbe425
1 file changed