encoding/json: remove unnecessary if conditions

Fixes gosimple warning "if err != nil { return err };
return nil' can be simplified to 'return err"

Change-Id: Ife7f78a3a76ab7802b5561d1afec536e103b504a
Reviewed-on: https://go-review.googlesource.com/108275


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>