Remove erroneous ,string struct tag

The JSON encoder was very upset with the ,string struct tag option. It
spat out the error:

json: invalid use of ,string struct tag, trying to unmarshal
"18aed9a7-dc83-406e-a4dc-40d53049541a" into string

The documentation appears to be incomplete on this struct tag option,
and removing it fixes this error.

chronograf.go

@@ -181,7 +181,7 @@ type Cell struct {
 
 // Layout is a collection of Cells for visualization
 type Layout struct {
-	ID          string `json:"id,string"`
+	ID          string `json:"id"`
 	Application string `json:"app"`
 	Measurement string `json:"measurement"`
 	Cells       []Cell `json:"cells"`