There’s some info out there you can find, but it all seems to be wrong in these later versions of Mediawiki.

We’re going to need to modify the AuthChangeFormFields hook. Lets add the following to our LocalSettings.php

function StripLogin(&$req, &$fi, &$fd, $a){
        #var_dump($fi);

        unset($fd['username']);
        unset($fd['password']);
        unset($fd["authmanager-userlogin-remembermypassword-help"]);
        unset($fd['userlogin-yourpassword']);
        unset($fd['userlogin-remembermypassword']);

        return true;
}

$wgHooks['AuthChangeFormFields'][] = 'StripLogin'

$wgAllowSiteCSSOnRestrictedPages = true;

I did still hide the login button with css, as it’s not part of this array.
You’ll put this in your MediaWiki:Common.css

#wpLoginAttempt{
 visibility: hidden;
}