Maybe remove custom style for datefield calendar

[douglasmiranda]

The current one is responsive.

[douglasmiranda]

Related files:

• https://github.com/douglasmiranda/django-admin-bootstrap/blob/master/bootstrap_admin/static/admin/css/forms.css
• https://github.com/django/django/blob/master/django/contrib/admin/static/admin/css/widgets.css
• https://github.com/django/django/blob/master/django/contrib/admin/static/admin/css/responsive.css