remove duplicate requires of mocha.

Mocha is already required by AS::TestCase, so remove the duplicate
requires.

activerecord/test/cases/helper.rb

@@ -5,7 +5,6 @@ require 'config'
 gem 'minitest'
 require 'minitest/autorun'
 require 'stringio'
-require 'mocha'
 
 require 'cases/test_case'
 require 'active_record'

activesupport/test/abstract_unit.rb

@@ -18,8 +18,6 @@ end
 require 'minitest/autorun'
 require 'empty_bool'
 
-silence_warnings { require 'mocha' }
-
 ENV['NO_RELOAD'] = '1'
 require 'active_support'
 

railties/test/generators_test.rb

@@ -1,7 +1,6 @@
 require 'generators/generators_test_helper'
 require 'rails/generators/rails/model/model_generator'
 require 'rails/generators/test_unit/model/model_generator'
-require 'mocha'
 
 class GeneratorsTest < Rails::Generators::TestCase
   include GeneratorsTestHelper