Remove finalize from NullSession

`finalize_session` should not be able to be called if a session has not
been started, we should never have a `NullSession` when we get there. So
we can remove the guard altogether to help make it obvious if this sort
of thing does happen when it's not supposed to.

Author: leequarella

activerecord/lib/active_record/asynchronous_queries_tracker.rb

@@ -7,9 +7,6 @@ class << self
         def active?
           true
         end
-
-        def finalize
-        end
       end
     end
 

activerecord/test/cases/adapter_test.rb

@@ -408,20 +408,6 @@ def test_async_query_foreground_fallback
     ensure
       ActiveSupport::Notifications.unsubscribe(subscriber) if subscriber
     end
-
-    def test_async_query_finalize_with_null_session
-      assert_nothing_raised do
-        @connection.select_all "SELECT * FROM posts", async: true
-      end
-
-      @connection.transaction do
-        assert_raises AsynchronousQueryInsideTransactionError do
-          @connection.select_all "SELECT * FROM posts", async: true
-        end
-      end
-    ensure
-      ActiveRecord::Base.asynchronous_queries_tracker.finalize_session
-    end
   end
 
   class AsynchronousQueriesTest < ActiveRecord::TestCase