Avoid accumulating transfer failure log files unless the assistant is being used.

Only the assistant uses these, and only the assistant cleans them up, so
make only git annex transferkeys write them,

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. Hope no-one depended on that old behavior.

Remote/Git.hs

@@ -386,7 +386,7 @@ copyFromRemote' r key file dest meterupdate
 					let go = copier
 #endif
 					runTransfer (Transfer Download u key)
-						file noRetry go
+						file noRetry noObserver go
 						<&&> checksuccess
 	| Git.repoIsSsh (repo r) = feedprogressback $ \feeder -> do
 		direct <- isDirect
@@ -502,7 +502,7 @@ copyToRemote' r key file p
 			( return True
 			, do
 				ensureInitialized
-				runTransfer (Transfer Download u key) file noRetry $ const $
+				runTransfer (Transfer Download u key) file noRetry noObserver $ const $
 					Annex.Content.saveState True `after`
 						Annex.Content.getViaTmpChecked (liftIO checksuccessio) key
 							(\d -> rsyncOrCopyFile params object d p)