luci-base: don't append object.method to the RPC url

It was a cosmetic feature that doesn't work anymore with the latest
uhttpd ubus API. Adding RESTful API resulted in treating URL part
following the /ubus/ as the new API request.

Suggested-by: Rafał Miłecki <rafal@milecki.pl>
Fixes: #4465
Signed-off-by: Jo-Philipp Wich <jo@mein.io>