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.

Upgrade/V1.hs

@@ -191,17 +191,16 @@ logFile1 repo key = Upgrade.V2.gitStateDir repo ++ keyFile1 key ++ ".log"
 
 lookupFile1 :: FilePath -> Annex (Maybe (Key, Backend Annex))
 lookupFile1 file = do
-	bs <- Annex.getState Annex.supportedBackends
 	tl <- liftIO $ try getsymlink
 	case tl of
 		Left _ -> return Nothing
-		Right l -> makekey bs l
+		Right l -> makekey l
 	where
 		getsymlink = do
 			l <- readSymbolicLink file
 			return $ takeFileName l
-		makekey bs l = do
-			case maybeLookupBackendName bs bname of
+		makekey l = do
+			case maybeLookupBackendName bname of
 				Nothing -> do
 					unless (null kname || null bname ||
 					        not (isLinkToAnnex l)) $