simplify remoteGetWorkerInfo logic

[rutsky]

retrieve "slave_commands" and "version" only when old buildslave is
detected.

[mention-bot]

By analyzing the blame information on this pull request, we identified @MichaelMayorov, @tardyp and @djmitche to be potential reviewers

[codecov-io]

Current coverage is 85.10%

Merging #2236 into master will increase coverage by <.01%

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+ Hits          27316      27317     +1   
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[djmitche]

So, for new workers the getWorkerInfo call includes all of this?

+1!

[rutsky]

@djmitche yes, new buildbot-worker (and latest versions of buildslave) get all info in single getWorkerInfo() (getSlaveInfo()) call.
I haven't noticed that when previously changed this function, so simplifying logic now.