remove doesPathExist check in checkKeyCheckSum

Before commit c565340adc,
it was statting the file in order to get its size, which was needed to
use an external hasher. In that commit since it no longer needed the
stat, I made it check doesPathExist since the old code implicitly
checked if the file existed. But that is not necessary, this should only
be ever called on files that exist.

Backend/Hash.hs

@@ -22,7 +22,6 @@ import Types.Backend
 import Types.KeySource
 import Utility.Hash
 import Utility.Metered
-import qualified Utility.RawFilePath as R
 
 import qualified Data.ByteString as S
 import qualified Data.ByteString.Short as S (toShort, fromShort)
@@ -125,14 +124,9 @@ keyValueE hash source meterupdate =
 
 checkKeyChecksum :: Hash -> Key -> RawFilePath -> Annex Bool
 checkKeyChecksum hash key file = catchIOErrorType HardwareFault hwfault $ do
-	fast <- Annex.getRead Annex.fast
-	exists <- liftIO $ R.doesPathExist file
-	case (exists, fast) of
-		(True, False) -> do
 	showAction (UnquotedString descChecksum)
 	sameCheckSum key 
 		<$> hashFile hash file nullMeterUpdate
-		_ -> return True
   where
 	hwfault e = do
 		warning $ UnquotedString $ "hardware fault: " ++ show e