Re-enable accumulating transfer failure log files for command-line actions

This was disabled in commit 61ccf95004,
because only the assistant used them, and they were clutter. But, now
--failed also uses them.

Remove the failure log files after successful transfers. Should avoid
most of the clutter problems.

Commit 61ccf95004 mentions a subtle behavior
change, which has now been reverted:

    There is one behavior change from this. If glacier is being used, and a
    manual git annex get --from glacier fails because the file isn't available
    yet, the assistant will no longer later see that failed transfer file and
    retry the get.

Annex/Transfer.hs

@@ -94,8 +94,12 @@ runTransfer' ignorelock t file shouldretry transferobserver transferaction = do
 			return observeFailure
 		else do
 			v <- retry info metervar $ transferaction meter
+			let ok = observeBool v
 			liftIO $ cleanup tfile lck
-			transferobserver (observeBool v) t info
+			if ok
+				then removeFailedTransfer t
+				else recordFailedTransfer t info
+			transferobserver ok t info
 			return v
   where
 #ifndef mingw32_HOST_OS