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.

Backend.hs

@@ -58,10 +58,7 @@ genKey source preferredbackend = do
 		Just k -> Just (makesane k, b)
   where
 	-- keyNames should not contain newline characters.
-	makesane k = k
-		{ keyName = S8.map fixbadchar (keyName k)
-		, keySerialization = Nothing
-		}
+	makesane k = k { keyName = S8.map fixbadchar (keyName k) }
 	fixbadchar c
 		| c == '\n' = '_'
 		| otherwise = c