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/Unannex.hs

@@ -13,10 +13,10 @@ import System.Directory
 import System.Posix.Files
 
 import Command
+import qualified Command.Drop
 import qualified Annex
 import qualified AnnexQueue
 import Utility
-import qualified Backend
 import LocationLog
 import Types
 import Content
@@ -33,7 +33,7 @@ seek = [withFilesInGit start]
 
 {- The unannex subcommand undoes an add. -}
 start :: CommandStartString
-start file = isAnnexed file $ \(key, backend) -> do
+start file = isAnnexed file $ \(key, _) -> do
 	ishere <- inAnnex key
 	if ishere
 		then do
@@ -46,13 +46,12 @@ start file = isAnnexed file $ \(key, backend) -> do
 				Annex.changeState $ \s -> s { Annex.force = True }
 
 			showStart "unannex" file
-			next $ perform file key backend
+			next $ perform file key
 		else stop
 
-perform :: FilePath -> Key -> Backend Annex -> CommandPerform
-perform file key backend = do
-	-- force backend to always remove
-	ok <- Backend.removeKey backend key (Just 0)
+perform :: FilePath -> Key -> CommandPerform
+perform file key = do
+	ok <- Command.Drop.dropKey key (Just 0) -- always remove
 	if ok
 		then next $ cleanup file key
 		else stop