Remove unused 'require minitest/mock'

minitest-mock has been extracted to a separate gem.
These two test files required the library but do not
use it.

Author: stefannibrasil

test/faker/default/test_faker_boolean.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require_relative '../../test_helper'
-require 'minitest/mock'
 
 class TestFakerBoolean < Test::Unit::TestCase
   def setup

test/faker/default/test_faker_number.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require_relative '../../test_helper'
-require 'minitest/mock'
 
 class TestFakerNumber < Test::Unit::TestCase
   def setup