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.

Types/Key.hs

@@ -23,7 +23,6 @@ data Key = Key
 	, keyMtime :: Maybe EpochTime
 	, keyChunkSize :: Maybe Integer
 	, keyChunkNum :: Maybe Integer
-	, keySerialization :: Maybe S.ByteString -- ^ cached serialization
 	} deriving (Eq, Ord, Read, Show)
 
 {- A filename may be associated with a Key. -}