remove/deprecate echoprints/puids

I would like to remove echoprint support from the documentation or maybe even from the code.
There is no point in supporting this, since this is dead technology on musicbrainz already. It only works on echoprint.musicbrainz.org and it isn't listed in the WS documentation.
At least I would mark it as deprecated.

Similarly, I would mark the PUID functions as deprecated or similar. They will still work until they are shut off on the server, but people should now that they shouldn't build on top of PUIDs anymore.