CDAP-451 Remove gateway related code

https://builds.cask.co/browse/CDAP-DUT74-2
Looks like the following can go too - gateway.max.cached.events.per.stream.num, gateway.max.cached.stream.events.bytes, gateway.max.cached.stream.events.num, gateway.stream.callback.exec.num.threads and gateway.stream.events.flush.interval.ms