Use truncate_all instead of DatabaseCleaner

Performance tests show both methods of truncating the database take
about the same time, so we can remove one dependency and we don't lose
anything in the process.

db/dev_seeds.rb

@@ -1,7 +1,4 @@
-unless Rails.env.test?
-  require "database_cleaner"
-  DatabaseCleaner.clean_with :truncation
-end
+ActiveRecord::Tasks::DatabaseTasks.truncate_all unless Rails.env.test?
 @logger = Logger.new(STDOUT)
 @logger.formatter = proc do |_severity, _datetime, _progname, msg|
                       msg unless @avoid_log