Remove unused `@txn` variable

This was added in c24c885209, removed in
b89ffe7f00, and then (unintentionally?)
reintroduced in 2d7ae1b08e.

activerecord/lib/active_record/core.rb

@@ -559,7 +559,6 @@ module ActiveRecord
         @marked_for_destruction   = false
         @destroyed_by_association = nil
         @new_record               = true
-        @txn                      = nil
         @_start_transaction_state = {}
         @transaction_state        = nil
       end

guides/source/debugging_rails_applications.md

@@ -606,7 +606,6 @@ You can also inspect for an object method this way:
 @new_record = true
 @readonly = false
 @transaction_state = nil
-@txn = nil
 ```
 
 You can also use `display` to start watching variables. This is a good way of