Remove useless assignments

Rubocop highlighted this. The todo list actually contains these files
but somehow the cop still runs and reports these as violations.

app/models/spree/taxon.rb

@@ -56,7 +56,7 @@ module Spree
 
     def pretty_name
       ancestor_chain = ancestors.inject("") do |name, ancestor|
-        name += "#{ancestor.name} -> "
+        name + "#{ancestor.name} -> "
       end
       ancestor_chain + name.to_s
     end

lib/spree/core/controller_helpers/common.rb

@@ -66,7 +66,7 @@ module Spree
           #   The layout to render can be set inside Spree configuration with the +:layout+ option.
           # Default layout is: +app/views/spree/layouts/spree_application+
           def get_layout
-            layout ||= Spree::Config[:layout]
+            Spree::Config[:layout]
           end
         end
       end