clean out dead code and refactor `ow_dl_*.erl`

[cmdrk]

There seems to be plenty of dead wood to clear out.

I finally figured out from reading the code:

$ curl localhost:4433/client/manifest                                                   
["libow4.gd","chat.proto","overworld.proto"]
$ curl localhost:4433/client/download > client.zip
$ mkdir client; mv client.zip client/
$ cd client
$ unzip client.zip
Archive:  client.zip
 extracting: libow.gd  
 extracting: chat.proto  
 extracting: overworld.proto  
$ ls
chat.proto      client.zip      libow.gd        overworld.proto

This is pretty un-ergonomic and unpleasant to work with. Let's come up with a nicer curl interface that provides the files but also gives a zip option for the programmatic client.