Remove "Confirm Email Address" field from registration form

RFC Topic: http://area51.phpbb.com/phpBB/viewtopic.php?f=84&t=38862

The registration form currently requires email address to be entered twice.

The confirmation field makes sense for passwords, because users do not see them as they enter them. Email addresses are displayed as they are entered and can be checked by visual inspection.

The proposal is to remove email address confirmation field.

After discussion in Pull Req and RFC, field has also been removed from UCP, ACP, and Installer.