Revert "cache the serialization of a Key"

This reverts commit 4536c93bb2.

That broke Read/Show of a Key, and unfortunately Key is read in at least
one place; the GitAnnexDistribution data type.

It would be worth bringing this optimisation back, but it would need
either a custom Read/Show instance that preserves back-compat, or
wrapping Key in a data type that contains the serialization, or changing
how GitAnnexDistribution is serialized.

Also, the Eq instance would need to compare keys with and without a
cached seralization the same.

Remote/Helper/Chunked.hs

@@ -68,10 +68,7 @@ chunkKeyStream :: Key -> ChunkSize -> ChunkKeyStream
 chunkKeyStream basek chunksize = ChunkKeyStream $ map mk [1..]
   where
 	mk chunknum = sizedk { keyChunkNum = Just chunknum }
-	sizedk = basek
-		{ keyChunkSize = Just (toInteger chunksize)
-		, keySerialization = Nothing
-		}
+	sizedk = basek { keyChunkSize = Just (toInteger chunksize) }
 
 nextChunkKeyStream :: ChunkKeyStream -> (Key, ChunkKeyStream)
 nextChunkKeyStream (ChunkKeyStream (k:l)) = (k, ChunkKeyStream l)