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.

Command/Add.hs

@@ -42,8 +42,8 @@ start pair@(file, _) = notAnnexed file $ do
 
 perform :: BackendFile -> CommandPerform
 perform (file, backend) = do
-	stored <- Backend.storeFileKey file backend
-	case stored of
+	k <- Backend.genKey file backend
+	case k of
 		Nothing -> stop
 		Just (key, _) -> do
 			moveAnnex key file