Remove unnecessary references to namespace in executor

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
There is no need to support multiple executor clusters from a scheduler, so the namespace of an executor is implicitly defined by the scheduler it connects to. See https://the-asf.slack.com/archives/C01QUFS30TD/p1618679585211100 for more context

Describe the solution you'd like
See apache/arrow#10084

[andygrove]

@edrevo fyi