revert the json i64/u64 support for now
revert the json i64/u64 support for now

infinityb/serde,Byron/serde,3Hren/serde,Hoverbear/serde,thorbenk/serde,JohnHeitmann/serde,skade/serde,sidred10/serde,csmulhern/serde,tempbottle/serde,pcwalton/serde,erickt/serde,Ticki/serde,serde-rs/serde,eefriedman/serde,jmesmon/serde,oli-obk/rust-serde