annex.backend is the new name for what was annex.backends

It takes a single key-value backend, rather than the unncessary and confusing list.
The old option still works if set.

Simplified some old old code too.

This commit was sponsored by Thomas Hochstein on Patreon.

Command/Migrate.hs

@@ -36,15 +36,13 @@ start file key = do
 		Nothing -> stop
 		Just oldbackend -> do
 			exists <- inAnnex key
-			newbackend <- choosebackend =<< chooseBackend file
+			newbackend <- maybe defaultBackend return 
+				=<< chooseBackend file
 			if (newbackend /= oldbackend || upgradableKey oldbackend key || forced) && exists
 				then do
 					showStart "migrate" file
 					next $ perform file key oldbackend newbackend
 				else stop
-  where
-	choosebackend Nothing = Prelude.head <$> orderedList
-	choosebackend (Just backend) = return backend
 
 {- Checks if a key is upgradable to a newer representation.
  -