luci-base: ui.js: avoid polling in a tight loop on confirm/rollback

The call function was based on the legacy XHR interface and it has not
been updated to comply with the new request interface which passes the
duration as part of the response object rather than as a separate
argument. This resulted in polling in a tight loop in certain cases
since the duration was undefined.

In addition there is no need to adjust apply_holdoff based on elapsed
time as no significant amount of time has elapsed at this point.

Signed-off-by: Erik Karlsson <erik.karlsson@genexis.eu>