luci-app-openvpn: fix stray uci permission warning

The OpenVPN file view uses a dummy Map() instance to render the breadcrumb
template which triggers a uci permission error since the view is being
rendered by a form() action which does not set up the expected permission
flags. CBI Map() instances should only be used for cbi() dispatch targets.

Solve the issue by appending the breadcrumb template directly to the
SimpleForm() instance and by removing the redundant dummy Map() instance.

Fixes: #4370
Signed-off-by: Jo-Philipp Wich <jo@mein.io>