RFR: Purge batched matrix cleanup

[Aleksey Shipilev]

On 10/14/2017 11:59 PM, Dominik Inführ wrote:
> The class ShenandoahMatrixCleanupTask seems to be not used anymore after
> this cleanup.

Yes, thanks for catching that! We'd purge it in another batch of cleanups:
  http://mail.openjdk.java.net/pipermail/shenandoah-dev/2017-October/004020.html

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Thanks,
-Aleksey