Removed outdated comments regarding `Slave::executorTerminated`.
Removed outdated comments regarding `Slave::executorTerminated`.

Review: https://reviews.apache.org/r/67901/

Gilbert88/mesos,gsantovena/mesos,Gilbert88/mesos,gsantovena/mesos,gsantovena/mesos,dforsyth/mesos,jpeach/mesos,andschwa/mesos,abudnik/mesos,Gilbert88/mesos,kaysoky/mesos,dforsyth/mesos,chhsia0/mesos,chhsia0/mesos,jpeach/mesos,reneploetz/mesos,abudnik/mesos,chhsia0/mesos,chhsia0/mesos,kaysoky/mesos,gsantovena/mesos,kaysoky/mesos,jpeach/mesos,kaysoky/mesos,dforsyth/mesos,kaysoky/mesos,jpeach/mesos,kaysoky/mesos,reneploetz/mesos,chhsia0/mesos,abudnik/mesos,abudnik/mesos,dforsyth/mesos,Gilbert88/mesos,andschwa/mesos,andschwa/mesos,andschwa/mesos,Gilbert88/mesos,reneploetz/mesos,dforsyth/mesos,dforsyth/mesos,gsantovena/mesos,reneploetz/mesos,reneploetz/mesos,abudnik/mesos,abudnik/mesos,reneploetz/mesos,andschwa/mesos,chhsia0/mesos,abudnik/mesos,dforsyth/mesos,gsantovena/mesos,jpeach/mesos,Gilbert88/mesos,gsantovena/mesos,jpeach/mesos,jpeach/mesos,andschwa/mesos,Gilbert88/mesos,chhsia0/mesos,reneploetz/mesos,andschwa/mesos,kaysoky/mesos