Remove depreacted webui_hostname and webui_port fields from SlaveInfo.

These have been deprecated since November 2013, at least Mesos 0.16.
Should be safe to remove at this point (People will have to upgrade
pre-0.16 to something between 0.16 and 0.20.1 inclusive, before
upgrading past 0.20.1).

Review: https://reviews.apache.org/r/26149