[SPARK-11163] Remove unnecessary addPendingTask calls.

This commit removes unnecessary calls to addPendingTask in
TaskSetManager.executorLost. These calls are unnecessary: for
tasks that are still pending and haven't been launched, they're
still in all of the correct pending lists, so calling addPendingTask
has no effect. For tasks that are currently running (which may still be
in the pending lists, depending on how they were scheduled), we call
addPendingTask in handleFailedTask, so the calls at the beginning
of executorLost are redundant.

I think these calls are left over from when we re-computed the locality
levels in addPendingTask; now that we call recomputeLocality separately,
I don't think these are necessary.

Now that those calls are removed, the readding parameter in addPendingTask
is no longer necessary, so this commit also removes that parameter.

markhamstra can you take a look at this?

cc vanzin

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #9154 from kayousterhout/SPARK-11163.