Remove "require 'thread'" statements

These are no longer required in the supported Ruby versions.

core/conditionvariable/broadcast_spec.rb

@@ -1,5 +1,4 @@
 require_relative '../../spec_helper'
-require 'thread'
 
 describe "ConditionVariable#broadcast" do
   it "releases all threads waiting in line for this resource" do

core/conditionvariable/marshal_dump_spec.rb

@@ -1,5 +1,4 @@
 require_relative '../../spec_helper'
-require 'thread'
 
 describe "ConditionVariable#marshal_dump" do
   it "raises a TypeError" do

core/conditionvariable/signal_spec.rb

@@ -1,5 +1,4 @@
 require_relative '../../spec_helper'
-require 'thread'
 
 describe "ConditionVariable#signal" do
   it "releases the first thread waiting in line for this resource" do

core/conditionvariable/wait_spec.rb

@@ -1,5 +1,4 @@
 require_relative '../../spec_helper'
-require 'thread'
 
 describe "ConditionVariable#wait" do
   it "calls #sleep on the given object" do

core/module/autoload_spec.rb

@@ -1,7 +1,6 @@
 require_relative '../../spec_helper'
 require_relative '../../fixtures/code_loading'
 require_relative 'fixtures/classes'
-require 'thread'
 
 describe "Module#autoload?" do
   it "returns the name of the file that will be autoloaded" do

core/process/fixtures/kill.rb

@@ -1,5 +1,3 @@
-require 'thread'
-
 pid_file = ARGV.shift
 scenario = ARGV.shift
 

library/objectspace/reachable_objects_from_spec.rb

@@ -38,7 +38,6 @@
   end
 
   it "finds an object stored in a Queue" do
-    require 'thread'
     o = Object.new
     q = Queue.new
     q << o
@@ -49,7 +48,6 @@
   end
 
   it "finds an object stored in a SizedQueue" do
-    require 'thread'
     o = Object.new
     q = SizedQueue.new(3)
     q << o