8u contains two copies of test/../FileUtils.java, one uses JDK9+ features

There are two copies of the test utility class FileUtils.java in the jdk8u-dev source tree:

```
$ find . -name FileUtils.java
./jdk/test/lib/testlibrary/jdk/testlibrary/FileUtils.java
./jdk/test/lib/jdk/test/lib/util/FileUtils.java
```

One of them is not used by anything. It also uses language features that are not present in 8u:

```
$ $JAVA_HOME/bin/javac ./jdk/test/lib/jdk/test/lib/util/FileUtils.java
# snip
./jdk/test/lib/jdk/test/lib/util/FileUtils.java:166: error: cannot infer type arguments for SimpleFileVisitor<T>
            java.nio.file.Files.walkFileTree(dir, new SimpleFileVisitor<>() {
                                                                       ^
  reason: cannot use '<>' with anonymous inner classes
```

The soluton for this case is simple, just remove the above file.

This is part of a wider issue of duplicated test material, but I'm filing for the one I found for now.