Remove deprecated config for 5.0

[wiibaa]

Meta issue => elastic/logstash#3858

[suyograo]

@wiibaa don't need to bump major version because 3.0.0 bump in master will only be packaged in LS 5.0

[suyograo]

@wiibaa travis was failing. Kicked it off again. Will check back in my morning tomorrow

[wiibaa]

@suyograo I can reproduce it locally for some test execution, running with debug, there is one event failing with an error like

{:timestamp=>"2016-05-27T22:27:19.016000+0200", :message=>"Error encoding event", :exception=>java.lang.ArrayIndexOutOfBoundsException: 7371, :event=>2016-05-27T20:27:14.213Z %{host} test, :level=>:warn, :file=>"logstash/outputs/redis.rb", :line=>"128", :method=>"receive"}

It seems the json codec is choking sometime, any clue ?
I got misleaded by the error message, nothing is wrong with the codec, need to investigate Stud:buffer and how it is used in this plugin, will keep you posted