Move `where` from `TreeManager` to `SelectManager`

`TreeManager` is inherited by `InsertManager` but `where` on
`InsertManager` doesn't work.

And also, remove `@ctx` since the instance variable is used only for the
`SelectManager`.

activerecord/lib/arel/delete_manager.rb

@@ -5,9 +5,7 @@ module Arel # :nodoc: all
     include TreeManager::StatementMethods
 
     def initialize(table = nil)
-      super()
       @ast = Nodes::DeleteStatement.new(table)
-      @ctx = @ast
     end
 
     def from(relation)

activerecord/lib/arel/insert_manager.rb

@@ -3,7 +3,6 @@
 module Arel # :nodoc: all
   class InsertManager < Arel::TreeManager
     def initialize(table = nil)
-      super()
       @ast = Nodes::InsertStatement.new(table)
     end
 

activerecord/lib/arel/select_manager.rb

@@ -7,7 +7,6 @@ module Arel # :nodoc: all
     STRING_OR_SYMBOL_CLASS = [Symbol, String]
 
     def initialize(table = nil)
-      super()
       @ast = Nodes::SelectStatement.new(table)
       @ctx = @ast.cores.last
     end
@@ -182,6 +181,14 @@ module Arel # :nodoc: all
       @ast.orders
     end
 
+    def where(expr)
+      if Arel::TreeManager === expr
+        expr = expr.ast
+      end
+      @ctx.wheres << expr
+      self
+    end
+
     def where_sql(engine = Table.engine)
       return if @ctx.wheres.empty?
 

activerecord/lib/arel/tree_manager.rb

@@ -40,10 +40,6 @@ module Arel # :nodoc: all
 
     attr_reader :ast
 
-    def initialize
-      @ctx = nil
-    end
-
     def to_dot
       collector = Arel::Collectors::PlainString.new
       collector = Visitors::Dot.new.accept @ast, collector
@@ -60,13 +56,5 @@ module Arel # :nodoc: all
       super
       @ast = @ast.clone
     end
-
-    def where(expr)
-      if Arel::TreeManager === expr
-        expr = expr.ast
-      end
-      @ctx.wheres << expr
-      self
-    end
   end
 end

activerecord/lib/arel/update_manager.rb

@@ -5,9 +5,7 @@ module Arel # :nodoc: all
     include TreeManager::StatementMethods
 
     def initialize(table = nil)
-      super()
       @ast = Nodes::UpdateStatement.new(table)
-      @ctx = @ast
     end
 
     ###