Inline the attachment form for taxon icons

There's no explicit support for multiple taxon attachment anywhere
else in the solidus codebase, and the attachment_partial_name was
using "paperclip" even for active_storage.

Given attachment adapters are substantially dormant prior to Rails 6.1
we just need to explain the change in the changelog without the burden
of an overcomplex deprecation cycle.

backend/app/views/spree/admin/taxons/_form.html.erb

@@ -20,7 +20,20 @@
       </div>
     <% end %>
 
-    <%= render "spree/admin/taxons/attachment_forms/#{f.object.attachment_partial_name}", f: f %>
+    <%= f.field_container :icon do %>
+      <%= f.label :icon %><br>
+      <%= f.file_field :icon %>
+      <% if f.object.icon_present? %>
+        <%= image_tag f.object.icon(:mini) %>
+        <%= link_to t('spree.actions.remove'),
+                    admin_taxonomy_taxon_attachment_path(@taxonomy,
+                                                         @taxon.id,
+                                                         attachment_definition: :icon,
+                                                         authenticity_token: form_authenticity_token),
+                    method: :delete,
+                    class: 'btn btn-sm btn-danger' %>
+      <% end %>
+    <% end %>
   </div>
 
   <div class="col-5">

core/app/models/spree/taxon/active_storage_attachment.rb

@@ -9,11 +9,5 @@ module Spree::Taxon::ActiveStorageAttachment
                    styles: { mini: '32x32>', normal: '128x128>' },
                    default_style: :mini
     validate :icon_is_an_image
-
-
-  end
-
-  def attachment_partial_name
-    'paperclip'
   end
 end

core/app/models/spree/taxon/paperclip_attachment.rb

@@ -19,10 +19,6 @@ def icon_present?
     icon.present?
   end
 
-  def attachment_partial_name
-    'paperclip'
-  end
-
   def destroy_attachment(definition)
     return false unless respond_to?(definition)