Remove workaround for Ruby 2.7 at ActiveSupport::Cache#lookup_store

Since the minimum required version is now 3.1,
this can be got rid of like the comment commands.

activesupport/lib/active_support/cache.rb

@@ -86,13 +86,7 @@ def lookup_store(store = nil, *parameters)
         case store
         when Symbol
           options = parameters.extract_options!
-          # clean this up once Ruby 2.7 support is dropped
-          # see https://github.com/rails/rails/pull/41522#discussion_r581186602
-          if options.empty?
-            retrieve_store_class(store).new(*parameters)
-          else
-            retrieve_store_class(store).new(*parameters, **options)
-          end
+          retrieve_store_class(store).new(*parameters, **options)
         when Array
           lookup_store(*store)
         when nil