Deprecate all_taxons

I can't find any reasonable circumstance this (untested) method would
ever ran in.

These are all very difficult to understand or use, but would be easy for
host applications to add in themselves if they wanted to continue using
them.

core/lib/spree/core/product_filters.rb

@@ -181,6 +181,7 @@ def self.taxons_below(taxon)
       #
       # idea: expand the format to allow nesting of labels?
       def self.all_taxons
+        Spree::Deprecation.warn "all_taxons is deprecated in solidus_core. Please add it to your own application to continue using it."
         taxons = Spree::Taxonomy.all.map { |t| [t.root] + t.root.descendants }.flatten
         {
           name:   'All taxons',

frontend/app/views/spree/shared/_filters.html.erb

@@ -1,4 +1,4 @@
-<% filters = @taxon ? @taxon.applicable_filters : [Spree::Core::ProductFilters.all_taxons] %>
+<% filters = @taxon.applicable_filters %>
 <% unless filters.empty? %>
   <%= form_tag '', method: :get, id: 'sidebar_products_search' do %>
     <%= hidden_field_tag 'per_page', params[:per_page] %>