[2.0] Remove deprecated/legacy code

REMOVED: All deprecated/legacy methods, modules, and classes have been removed.

_Code supporting no-longer-extant parts of the Facebook API_:
* REST API
* Exchanging session keys for tokens
* OAuth#get_user_from_cookies (since you can only redeem the code once) 

_Old Koala structures_:
* GraphAPI, RestAPI, and GraphAndRestAPI modules
* #graph_api accessor for RealtimeUpdates and TestUsers
* TyphoeusService and NetHTTPService and associated accessors
* A score of deprecated HTTPService options