Make where_sql more few code

To be honest, it is hardly worth having a dedicated method for
`where_sql`, since it is a reduced version of `to_sql`.

I've left it for now, but just made it more few code (used `to_sql`).

activerecord/lib/active_record/relation/finder_methods.rb

@@ -328,8 +328,8 @@ def exists?(conditions = :none)
     # the expected number of results should be provided in the +expected_size+
     # argument.
     def raise_record_not_found_exception!(ids = nil, result_size = nil, expected_size = nil, key = primary_key, not_found_ids = nil) # :nodoc:
-      conditions = arel.where_sql(@klass)
-      conditions = " [#{conditions}]" if conditions
+      conditions = " [#{arel.where_sql(klass)}]" unless where_clause.empty?
+
       name = @klass.name
 
       if ids.nil?

activerecord/lib/arel/select_manager.rb

@@ -186,8 +186,7 @@ def orders
     def where_sql(engine = Table.engine)
       return if @ctx.wheres.empty?
 
-      viz = Visitors::WhereSql.new(engine.connection.visitor, engine.connection)
-      Nodes::SqlLiteral.new viz.accept(@ctx, Collectors::SQLString.new).value
+      Nodes::SqlLiteral.new("WHERE #{Nodes::And.new(@ctx.wheres).to_sql(engine)}")
     end
 
     def union(operation, other = nil)

activerecord/lib/arel/visitors.rb

@@ -5,7 +5,6 @@
 require "arel/visitors/sqlite"
 require "arel/visitors/postgresql"
 require "arel/visitors/mysql"
-require "arel/visitors/where_sql"
 require "arel/visitors/dot"
 
 module Arel # :nodoc: all

activerecord/lib/arel/visitors/where_sql.rb

@@ -1,22 +0,0 @@
-# frozen_string_literal: true
-
-module Arel # :nodoc: all
-  module Visitors
-    class WhereSql < Arel::Visitors::ToSql
-      def initialize(inner_visitor, *args, &block)
-        @inner_visitor = inner_visitor
-        super(*args, &block)
-      end
-
-      private
-        def visit_Arel_Nodes_SelectCore(o, collector)
-          collector << "WHERE "
-          wheres = o.wheres.map do |where|
-            Nodes::SqlLiteral.new(@inner_visitor.accept(where, collector.class.new).value)
-          end
-
-          inject_join wheres, collector, " AND "
-        end
-    end
-  end
-end