Remove formattable from webhook actions.

backend/extensions/Squidex.Extensions/Actions/Webhook/WebhookAction.cs

@@ -44,7 +44,6 @@ namespace Squidex.Extensions.Actions.Webhook
 
         [Display(Name = "Headers (Optional)", Description = "The message headers in the format '[Key]=[Value]', one entry per line.")]
         [Editor(RuleFieldEditor.TextArea)]
-        [Formattable]
         public string Headers { get; set; }
 
         [Display(Name = "Shared Secret", Description = "The shared secret that is used to calculate the payload signature.")]