You can change this by editing the page template for the account activation form. Your website needs to have a custom theme to be able to do this. 

Go to Website > [your site] > Theme. Under your current custom theme, look for the Account Activation template. It has the file name users_activations_new_wide.html. Here you can edit the code to remove the fields you do not wish to display.

If you don't have a custom theme yet, you can find the instructions for creating one here.