build-infra: Cleanup of misc changes in build-infra

There are a couple of minor changes in build-infra that haven't been merged to jdk8/build yet. All are small and insignificant on their own.

* Removal of redundant package entries in jar exclude lists
* Removal of unnecessary source file in native compilation
* Cleaned up comment about code exclusion in CompileJavaClasses.gmk

Not urgent but would be convenient to get in to reduce diff between jdk8/build and build-infra/jdk8.