[11] Review request: JDK-8199071: Fix gradle deprecation warnings

[Kevin Rushforth]

Phil,

Please review the following:

https://bugs.openjdk.java.net/browse/JDK-8199071
http://cr.openjdk.java.net/~kcr/8199071/webrev.00/

This is mostly simple cleanup. The only substantive change was to remove 
the override of the deprecated sourceSets.main.output.classesDir 
property (added by the fix for 
https://bugs.openjdk.java.net/browse/JDK-8185358 so we could transition 
from gradle 3 to gradle 4), along with associated changes to use 
gradle's per-language output directory.

-- Kevin