PLT-2939 Fix Default slash command icon is invalid (#3250)

model/incoming_webhook.go

@@ -13,7 +13,6 @@ import (
 
 const (
 	DEFAULT_WEBHOOK_USERNAME = "webhook"
-	DEFAULT_WEBHOOK_ICON     = "/static/images/webhook_icon.jpg"
 )
 
 type IncomingWebhook struct {