MAINTAINERS: remove ericchiang@

I haven't had time to contribute to dex recently and it's been over a
year since I've worked on Kubernetes. Going to make this official and
remove myself from the MAINTAINERS list. I'll still be around if you
need to know why any of the code is so crazy :)

MAINTAINERS

 Rithu John <rithujohn191@gmail.com> (@rithujohn191)
-Eric Chiang <eric.chiang.m@gmail.com> (@ericchiang)
 Stephan Renatus <srenatus@chef.io> (@srenatus)
 Joel Speed <Joel.speed@hotmail.co.uk> (@JoelSpeed)