RFR: 8149773: StandardDocFileFactory should be converted to use java.nio.file.Path

[Jonathan Gibbons]

Please review this cleanup to convert the javadoc DocFile impl class to 
use java.nio.file.Path instead of java.io.File.
Some related dead code is also deleted.

webrev: http://cr.openjdk.java.net/~jjg/8149773/webrev.00/
JBS: https://bugs.openjdk.java.net/browse/JDK-8149773
-- Jon