Remove stanford_theme_enabled from themes templates

themes/edx.org/lms/templates/emails/activation_email.txt

-<%namespace file="../main.html" import="stanford_theme_enabled" />
 <%! from django.utils.translation import ugettext as _ %>
 ${_("Thank you for signing up for {platform_name}.").format(platform_name=settings.PLATFORM_NAME)}
 

themes/red-theme/lms/templates/header.html

 ## mako
 <%namespace name='static' file='static_content.html'/>
-<%namespace file='main.html' import="login_query, stanford_theme_enabled"/>
+<%namespace file='main.html' import="login_query"/>
 <%!
 from django.core.urlresolvers import reverse
 from django.utils.translation import ugettext as _