Remove Unused require

It looks like they are left from old design

lib/arel/nodes/node.rb

@@ -1,6 +1,4 @@
 # frozen_string_literal: true
-require 'arel/collectors/sql_string'
-
 module Arel
   module Nodes
     ###

lib/arel/select_manager.rb

@@ -1,6 +1,4 @@
 # frozen_string_literal: true
-require 'arel/collectors/sql_string'
-
 module Arel
   class SelectManager < Arel::TreeManager
     include Arel::Crud

lib/arel/tree_manager.rb

@@ -1,6 +1,4 @@
 # frozen_string_literal: true
-require 'arel/collectors/sql_string'
-
 module Arel
   class TreeManager
     include Arel::FactoryMethods

lib/arel/visitors/to_sql.rb

@@ -1,7 +1,4 @@
 # frozen_string_literal: true
-require 'bigdecimal'
-require 'date'
-
 module Arel
   module Visitors
     class UnsupportedVisitError < StandardError

test/collectors/test_bind.rb

@@ -1,8 +1,6 @@
 # frozen_string_literal: true
 require 'helper'
 
-require 'arel/collectors/bind'
-
 module Arel
   module Collectors
     class TestBind < Arel::Test

test/collectors/test_composite.rb

@@ -3,7 +3,6 @@
 
 require 'arel/collectors/bind'
 require 'arel/collectors/composite'
-require 'arel/collectors/sql_string'
 
 module Arel
   module Collectors

test/helper.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 require 'rubygems'
 require 'minitest/autorun'
-require 'fileutils'
 require 'arel'
 
 require 'support/fake_record'

test/nodes/test_binary.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 require 'helper'
-require 'set'
 
 module Arel
   module Nodes

test/nodes/test_table_alias.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 require 'helper'
-require 'ostruct'
 
 module Arel
   module Nodes

test/visitors/test_depth_first.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 require 'helper'
-require 'set'
 
 module Arel
   module Visitors

test/visitors/test_to_sql.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 require 'helper'
-require 'set'
+require 'bigdecimal'
 
 module Arel
   module Visitors