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/Uninit.hs

@@ -44,7 +44,7 @@ seek ps = do
 
 {- git annex symlinks that are not checked into git could be left by an
  - interrupted add. -}
-startCheckIncomplete :: FilePath -> (Key, Backend) -> CommandStart
+startCheckIncomplete :: FilePath -> Key -> CommandStart
 startCheckIncomplete file _ = error $ unlines
 	[ file ++ " points to annexed content, but is not checked into git."
 	, "Perhaps this was left behind by an interrupted git annex add?"