Remove references to any public Staticman API instance (#775)

assets/js/staticman.js

@@ -11,7 +11,7 @@ layout: null
     $(form).addClass('disabled');
 
     {% assign sm = site.staticman -%}
-    var endpoint = '{{ sm.endpoint | default: "https://staticman3.herokuapp.com/v3/entry/github/" }}';
+    var endpoint = '{{ sm.endpoint }}';
     var repository = '{{ sm.repository }}';
     var branch = '{{ sm.branch }}';