Try this,
Joomla default login module is protected. So you can't edit/disable it from admin side.
Just check extensions-> extension manager -> Search for login
Then that module will display. but you can't make it disable.
So the solution for override this feature without touching core files is template override.
You can simply override this view index.php?option=com_users&view=login
in your template.
Editing Protected extension via DB tables
The extensions can not be edited, but you can manage it by turning it
ON, or OFF. Protected, mean that this extension can not be managed,
otherwise it will broke structure of your site. However, if you wish
to bring extension to unlocked status, you can access your DB (in my
case MySQL edited by phpMyAdmin), find reliable table of structure,
find desired string, and change "Status" from "1" to "0". Usually your
host providing you with some DB administering tools.
Hope it helps..