replace (Key, Backend) with Key

Only fsck and reinject and the test suite used the Backend, and they can
look it up as needed from the Key. This simplifies the code and also speeds
it up.

There is a small behavior change here. Before, all commands would warn when
acting on an annexed file with an unknown backend. Now, only fsck and
reinject show that warning.

Command/Copy.hs

@@ -30,9 +30,9 @@ seek ps = do
 {- A copy is just a move that does not delete the source file.
  - However, --auto mode avoids unnecessary copies, and avoids getting or
  - sending non-preferred content. -}
-start :: Maybe Remote -> Maybe Remote -> FilePath -> (Key, Backend) -> CommandStart
-start to from file (key, backend) = stopUnless shouldCopy $ 
-	Command.Move.start to from False file (key, backend)
+start :: Maybe Remote -> Maybe Remote -> FilePath -> Key -> CommandStart
+start to from file key = stopUnless shouldCopy $ 
+	Command.Move.start to from False file key
   where
 	shouldCopy = checkAuto (check <||> numCopiesCheck file key (<))
 	check = case to of