Cleanup dead code for Ruby < 2.2

Gemfile

@@ -12,7 +12,7 @@ gem 'concurrent-ruby-edge', Concurrent::EDGE_VERSION, options
 gem 'concurrent-ruby-ext', Concurrent::VERSION, options.merge(platform: :mri)
 
 group :development do
-  gem 'rake', (Concurrent.ruby_version :<, 2, 2, 0) ? '~> 12.0' : '~> 13.0'
+  gem 'rake', '~> 13.0'
   gem 'rake-compiler', '~> 1.0', '>= 1.0.7'
   gem 'rake-compiler-dock', '~> 1.0'
   gem 'pry', '~> 0.11', platforms: :mri

Rakefile

@@ -2,15 +2,6 @@ require_relative 'lib/concurrent-ruby/concurrent/version'
 require_relative 'lib/concurrent-ruby-edge/concurrent/edge/version'
 require_relative 'lib/concurrent-ruby/concurrent/utility/engine'
 
-if Concurrent.ruby_version :<, 2, 0, 0
-  # @!visibility private
-  module Kernel
-    def __dir__
-      File.dirname __FILE__
-    end
-  end
-end
-
 core_gemspec = Gem::Specification.load File.join(__dir__, 'concurrent-ruby.gemspec')
 ext_gemspec  = Gem::Specification.load File.join(__dir__, 'concurrent-ruby-ext.gemspec')
 edge_gemspec = Gem::Specification.load File.join(__dir__, 'concurrent-ruby-edge.gemspec')

lib/concurrent-ruby-edge/concurrent-edge.rb

@@ -6,7 +6,7 @@
 require 'concurrent/agent'
 require 'concurrent/channel'
 require 'concurrent/lazy_register'
-require 'concurrent/executor/wrapping_executor' if Concurrent.ruby_version :>=, 2, 1, 0
+require 'concurrent/executor/wrapping_executor'
 
 require 'concurrent/edge/lock_free_linked_set'
 require 'concurrent/edge/lock_free_queue'
@@ -16,4 +16,4 @@
 require 'concurrent/edge/channel'
 
 require 'concurrent/edge/processing_actor'
-require 'concurrent/edge/erlang_actor' if Concurrent.ruby_version :>=, 2, 1, 0
+require 'concurrent/edge/erlang_actor'

lib/concurrent-ruby-edge/concurrent/edge/erlang_actor.rb

@@ -1,7 +1,3 @@
-if Concurrent.ruby_version :<, 2, 1, 0
-  raise 'ErlangActor requires at least ruby version 2.1'
-end
-
 module Concurrent
 
   # This module provides actor abstraction that has same behaviour as Erlang actor.

lib/concurrent-ruby/concurrent/synchronization/lockable_object.rb

@@ -4,9 +4,7 @@ module Synchronization
     # @!visibility private
     # @!macro internal_implementation_note
     LockableObjectImplementation = case
-                                   when Concurrent.on_cruby? && Concurrent.ruby_version(:<=, 1, 9, 3)
-                                     MonitorLockableObject
-                                   when Concurrent.on_cruby? && Concurrent.ruby_version(:>, 1, 9, 3)
+                                   when Concurrent.on_cruby?
                                      MutexLockableObject
                                    when Concurrent.on_jruby?
                                      JRubyLockableObject