Remove redundant placeholders in attachment fields

Saying that we're supposed to introduce a descriptive title in a field
labelled as "Title" is redundant. Besides, the text of the placeholder
was barely distinguishable, making it harder to fill in the form.

app/components/attachable/fields_component.html.erb

@@ -4,7 +4,7 @@
   <%= f.hidden_field :cached_attachment %>
 
   <div class="small-12 column title">
-    <%= f.text_field :title, placeholder: t("#{plural_name}.form.title_placeholder") %>
+    <%= f.text_field :title %>
   </div>
 
   <% if attachable.attachment.attached? && attachable.attachment.image? %>