command/views: Remove unused drift summary message

This was dead code, and there is no clear way to retrieve this
information, as we currently only derive the drift information as part
of the rendering process.

internal/command/views/json/message_types.go

@@ -11,7 +11,6 @@ const (
 	// Operation results
 	MessagePlannedChange MessageType = "planned_change"
 	MessageChangeSummary MessageType = "change_summary"
-	MessageDriftSummary  MessageType = "drift_summary"
 	MessageOutputs       MessageType = "outputs"
 
 	// Hook-driven messages

internal/command/views/json_view.go

@@ -103,14 +103,6 @@ func (v *JSONView) ChangeSummary(cs *json.ChangeSummary) {
 	)
 }
 
-func (v *JSONView) DriftSummary(cs *json.ChangeSummary) {
-	v.log.Info(
-		cs.String(),
-		"type", json.MessageDriftSummary,
-		"changes", cs,
-	)
-}
-
 func (v *JSONView) Hook(h json.Hook) {
 	v.log.Info(
 		h.String(),