Cache default country simply in memory

This simplifies the code, avoids a method naming collision with Rails
and should be faster as well.

app/models/spree/country.rb

@@ -6,12 +6,6 @@ module Spree
 
     validates :name, :iso_name, presence: true
 
-    def self.cached_find_by(attrs)
-      Rails.cache.fetch("countries/#{attrs.hash}", expires_in: 1.hour) do
-        find_by(attrs)
-      end
-    end
-
     def <=>(other)
       name <=> other.name
     end

app/services/default_country.rb

@@ -10,7 +10,10 @@ class DefaultCountry
   end
 
   def self.country
-    Spree::Country.cached_find_by(iso: ENV.fetch("DEFAULT_COUNTRY_CODE",
-                                                 nil)) || Spree::Country.first
+    # Changing ENV requires restarting the process.
+    iso = ENV.fetch("DEFAULT_COUNTRY_CODE", nil)
+
+    # When ENV changes on restart, this cache will be reset as well.
+    @country ||= Spree::Country.find_by(iso:) || Spree::Country.first
   end
 end