Improve label in dashboard actions link form

We were using a placeholder to indicate content which should be part of
a label.

It might be better to actually use "Link address" as a label instead of
"URL". I'm using "URL" because it's used in other places in the admin
section.

config/locales/es/general.yml

@@ -952,8 +952,6 @@ es:
       title: Enlaces
       note: Añade los enlaces que sean de interés.
       add_new_link: Añadir nuevo enlace
-    link_fields:
-      url_placeholder: Dirección del enlace
   communities:
     show:
       surveys: Encuestas