Revert "Use active record cache when available"

This reverts commit a27d692656.

The reason for reverting is that this middleware has no effect. In Rails
5.0+, Sidekiq runs its jobs using the Rails reloader/executor API[1].

In Rails 5.0+, however, the ActiveRecord query cache registers its own
hooks with the executor[2], and these hooks automatically enable the
query cache for the duration of any work the executor performs[3].

As a result, the query cache is already on by default for any Sidekiq
jobs run in Rails 5.0+, so it's not necessary to separately enable it in
a middleware.

[1] eca6acc0ce/lib/sidekiq/rails.rb (L33-L37)
[2] f8c00c1300/activerecord/lib/active_record/railtie.rb (L159-L163)
[3] f8c00c1300/activerecord/lib/active_record/query_cache.rb (L26-L41)

lib/sidekiq/middleware/server/active_record_cache.rb

@@ -1,11 +0,0 @@
-module Sidekiq
-  module Middleware
-    module Server
-      class ActiveRecordCache
-        def call(*args, &block)
-          ::ActiveRecord::Base.cache(&block)
-        end
-      end
-    end
-  end
-end

lib/sidekiq/rails.rb

@@ -9,15 +9,10 @@ module Sidekiq
     # class block. Definitely before config/environments/*.rb and
     # config/initializers/*.rb.
     config.before_configuration do
-      if defined?(::ActiveRecord)
+      if ::Rails::VERSION::MAJOR < 5 && defined?(::ActiveRecord)
         Sidekiq.server_middleware do |chain|
-          if ::Rails::VERSION::MAJOR < 5
-            require 'sidekiq/middleware/server/active_record'
-            chain.add Sidekiq::Middleware::Server::ActiveRecord
-          end
-
-          require 'sidekiq/middleware/server/active_record_cache'
-          chain.add Sidekiq::Middleware::Server::ActiveRecordCache
+          require 'sidekiq/middleware/server/active_record'
+          chain.add Sidekiq::Middleware::Server::ActiveRecord
         end
       end
     end
@@ -59,4 +54,4 @@ if defined?(::Rails) && ::Rails::VERSION::MAJOR < 4
   $stderr.puts("**************************************************")
   $stderr.puts("⛔️ WARNING: Sidekiq server is no longer supported by Rails 3.2 - please ensure your server/workers are updated")
   $stderr.puts("**************************************************")
-end
+end