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
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| 2h 11m 35s |