Remove duplicate date_epoch

Author: nobu

Rakefile

@@ -7,11 +7,6 @@ Rake::TestTask.new(:test) do |t|
   t.test_files = FileList['test/**/test_*.rb']
 end
 
-task "build" => "date_epoch"
-task "date_epoch" do
-  ENV["SOURCE_DATE_EPOCH"] = IO.popen(%W[git -C #{__dir__} log -1 --format=%ct], &:read).chomp
-end
-
 ENV['RUBYOPT'] = "-w"
 
 task :default => [:test]