Hi Pookiefeb,
I just went through this again on my test account to verify that this works and it does. You must either be editing the wrong action, removing the incorrect field or your customization to the screen is preventing this fix from working. If you really need this to work, I suggest you copy the stock screen and replace your current new activity screen with the copy. You can then follow the steps I provided and the screen will function the way you desire.
FYI: You can check the name of the screen that is currently in use by hovering your cursor over the link.
-Audisho