Fixed an issue where task executor IDs are missing in the master.
Fixed an issue where task executor IDs are missing in the master.

Due to a bug, the agent was erroneously clearing the executor ID
of non-command executor's tasks before sending the
`ReregisterSlaveMessage` message. This leads to the master having
tasks with missing executor IDs (see MESOS-8135).

Also, it turns out that the clearing of executor ID's is actually
unnecessary altogether, as command executor tasks already do not
have an executor ID set.

Review: https://reviews.apache.org/r/63353/

chhsia0/mesos,asamerh4/mesos,chhsia0/mesos,abudnik/mesos,andschwa/mesos,kaysoky/mesos,andschwa/mesos,gsantovena/mesos,asamerh4/mesos,chhsia0/mesos,reneploetz/mesos,Gilbert88/mesos,abudnik/mesos,jpeach/mesos,jpeach/mesos,dforsyth/mesos,andschwa/mesos,gsantovena/mesos,jpeach/mesos,andschwa/mesos,dforsyth/mesos,asamerh4/mesos,dforsyth/mesos,andschwa/mesos,reneploetz/mesos,Gilbert88/mesos,Gilbert88/mesos,abudnik/mesos,andschwa/mesos,gsantovena/mesos,chhsia0/mesos,asamerh4/mesos,kaysoky/mesos,jpeach/mesos,kaysoky/mesos,gsantovena/mesos,chhsia0/mesos,jpeach/mesos,gsantovena/mesos,Gilbert88/mesos,dforsyth/mesos,Gilbert88/mesos,reneploetz/mesos,kaysoky/mesos,dforsyth/mesos,andschwa/mesos,abudnik/mesos,kaysoky/mesos,abudnik/mesos,Gilbert88/mesos,kaysoky/mesos,asamerh4/mesos,reneploetz/mesos,asamerh4/mesos,chhsia0/mesos,jpeach/mesos,dforsyth/mesos,chhsia0/mesos,reneploetz/mesos,gsantovena/mesos,reneploetz/mesos,abudnik/mesos,kaysoky/mesos,Gilbert88/mesos,reneploetz/mesos,gsantovena/mesos,jpeach/mesos,asamerh4/mesos,abudnik/mesos,dforsyth/mesos