In ClosingFuture, remove a workaround for [JDK-7101822](https://bugs.openjdk.java.net/browse/JDK-7101822)(?) now that we [build with a JDK after JDK 8](https://github.com/google/guava/issues/6549). This follows up on cl/332225001 and cl/643394004.

In ClosingFuture, remove a workaround for [JDK-7101822](https://bugs.openjdk.java.net/browse/JDK-7101822)(?) now that we [build with a JDK after JDK 8](https://github.com/google/guava/issues/6549). This follows up on cl/332225001 and cl/643394004. #3998