Revert "Adjust scheduler isCancelled, see #2903"

* Reverting this for 2.1.x since someone might rely on
  the old behavior of always getting isCancelled == true
  after calling cancel

This reverts commit 0c93ca317ef9085c3928e761843aa6d60912741c.
Merge pull request #1167 from akka/wip-2903-revert-patriknw

Revert "Adjust scheduler isCancelled, see #2903"