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.

app/views/links/_link_fields.html.erb

@@ -1,8 +1,7 @@
 <div id="<%= dom_id(f.object) %>" class="nested-fields">
   <div class="small-12 column attachment-actions">
     <%= f.text_field :label %>
-
-    <%= f.text_field :url, placeholder: t("links.link_fields.url_placeholder") %>
+    <%= f.text_field :url %>
 
     <div class="action-remove text-right">
       <%= render_destroy_element_link(f, f.object) %>