By default, the username is asked to the user during the registration process. If you remove the "username" field from the registration form as you apparently did, then it's the email which is used as username for Joomla (because Joomla requires a username for each user.
Unfortunately, there is no possibility to remove the username field in the password reminder form of Joomla. What you can do though is to change the text of the field to "email" instead of "username" with a joomla translation override:
www.inmotionhosting.com/support/edu/joom...ew-language-override