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/FromKey.hs

@@ -15,7 +15,6 @@ import Control.Monad (unless)
 import Command
 import qualified AnnexQueue
 import Utility
-import qualified Backend
 import Content
 import Messages
 import Types.Key
@@ -30,7 +29,7 @@ seek = [withFilesMissing start]
 start :: CommandStartString
 start file = notBareRepo $ do
 	key <- cmdlineKey
-	inbackend <- Backend.hasKey key
+	inbackend <- inAnnex key
 	unless inbackend $ error $
 		"key ("++keyName key++") is not present in backend"
 	showStart "fromkey" file