Don't use reflection to call Java 6 methods in FileUtil

The FileUtil class uses reflection to call the following java.io.File methods:

setWritable(boolean, boolean)
setReadable(boolean, boolean)
setExecutable(boolean, boolean)

Reflection was used because the methods were introduced in Java 6, and the code had to run on older platforms. Now Java 6 is the lowest supported platform, so we can call the methods directly.