Revert "use existing debug machinery for explain"

This reverts commit 409572c9e4.

Messages.hs

@@ -50,7 +50,6 @@ module Messages (
 	outputMessage,
 	withMessageState,
 	MessageState,
-	explain,
 	prompt,
 	mkPrompter,
 	sanitizeTopLevelExceptionMessages,
@@ -282,11 +281,6 @@ debugDisplayer = do
 		S.hPutStr stderr (safeOutput s <> "\n")
 		hFlush stderr
 
-explain :: Maybe String -> String -> Annex ()
-explain Nothing _ = return ()
-explain (Just f) msg = fastDebug' "explain" $ 
-	RawDebugMessage ('[' : f ++ " " ++ msg ++ "]")
-
 {- Should commands that normally output progress messages have that
  - output disabled? -}
 commandProgressDisabled :: Annex Bool