remove unused backend machinery

The only remaining vestiage of backends is different types of keys. These
are still called "backends", mostly to avoid needing to change user interface
and configuration. But everything to do with storing keys in different
backends was gone; instead different types of remotes are used.

In the refactoring, lots of code was moved out of odd corners like
Backend.File, to closer to where it's used, like Command.Drop and
Command.Fsck. Quite a lot of dead code was removed. Several data structures
became simpler, which may result in better runtime efficiency. There should
be no user-visible changes.

Annex.hs

@@ -34,7 +34,6 @@ type Annex = StateT AnnexState IO
 data AnnexState = AnnexState
 	{ repo :: Git.Repo
 	, backends :: [Backend Annex]
-	, supportedBackends :: [Backend Annex]
 	, remotes :: [Remote Annex]
 	, repoqueue :: Queue
 	, quiet :: Bool
@@ -52,12 +51,11 @@ data AnnexState = AnnexState
 	, cipher :: Maybe Cipher
 	}
 
-newState :: [Backend Annex] -> Git.Repo -> AnnexState
-newState allbackends gitrepo = AnnexState
+newState :: Git.Repo -> AnnexState
+newState gitrepo = AnnexState
 	{ repo = gitrepo
 	, backends = []
 	, remotes = []
-	, supportedBackends = allbackends
 	, repoqueue = empty
 	, quiet = False
 	, force = False
@@ -75,9 +73,8 @@ newState allbackends gitrepo = AnnexState
 	}
 
 {- Create and returns an Annex state object for the specified git repo. -}
-new :: Git.Repo -> [Backend Annex] -> IO AnnexState
-new gitrepo allbackends =
-	newState allbackends `liftM` (liftIO . Git.configRead) gitrepo
+new :: Git.Repo -> IO AnnexState
+new gitrepo = newState `liftM` (liftIO . Git.configRead) gitrepo
 
 {- performs an action in the Annex monad -}
 run :: AnnexState -> Annex a -> IO (a, AnnexState)