Go to the menu, select the DNA item, then find the sixth sequence and click on the fourth memory. (The name of the mission is “Small Assignment”.)
After the introductory video to the mission, you will, as always, be offered to either confirm the mission or reject it. This time, you choose “reject”.
Exit replay mode and go to the nearest blacksmith and buy any armor from him.
Voila! Ezio is now without a hood.
To put the hood back on, you must complete the above mission completely.
Have a good game!