encoding/json: cleanup leftover redundant variables.
encoding/json: cleanup leftover redundant variables.

Those variables come from ancient times when reflect.Value was
an interface.

R=dave, rsc
CC=golang-dev
https://codereview.appspot.com/6946059

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang