remove silverstripe/admin dependency

[lekoala]

At the moment, there is this line in the template

silverstripe-login-forms/themes/login-forms/templates/Security.ss

Line 12 in e0882c2

<% require css("silverstripe/admin: client/dist/styles/bundle.css") %>

which means the default template doesn't work without the silverstripe/admin module. Since it isn't specified in the composer.json file, I think it would be nice if these styles were made optional. I've removed that line and it doesn't seem to have much impact anyway on the layout.