Removing old etherpad code

I'm currently looking into revamping our settings-system to understand the concept of scopes (`get`, `global`, `local` and possibly more in the future). So I have to adapt everything in etherpad to the new system. While searching for usages for the settings-array I found two chunks of old etherpad code, which are not used anymore.

* `pad_docbar.js` contained options like changing the pad-title or the password in the Web-UI
* `isProPad` was used to determine if a pad is public in the **old** etherpad. Its value is always [false](https://github.com/ether/etherpad-lite/blob/master/src/node/handler/PadMessageHandler.js#L1048), since all pads are public, except for group-pads which are not affected by `isProPad` (tested that).

These two changes are split into two commits, so if there are concerns about removing one of them I can split this up into two pull-requests.

Things I tested prior submitting that:
* access to group pads
* setting a password on group pads via the API
* timeslider
* basic pad-editing and readonly-links
* all unit tests