Remove GCE cloud setting from AppVeyor config
Remove GCE cloud setting from AppVeyor config

AppVeyor has informed us that this may no longer be necessary after some
infrastructure upgrades on their side, so let's see how this goes!

graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust