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/Hash.hs

@@ -181,7 +181,6 @@ trivialMigrate' oldkey newbackend afile maxextlen
 	| migratable && hasExt oldvariety = Just $ oldkey
 		{ keyName = keyHash oldkey
 		, keyVariety = newvariety
-		, keySerialization = Nothing
 		}
 	{- Fast migration from hash to hashE backend. -}
 	| migratable && hasExt newvariety = case afile of
@@ -190,7 +189,6 @@ trivialMigrate' oldkey newbackend afile maxextlen
 			{ keyName = keyHash oldkey 
 				<> encodeBS (selectExtension maxextlen file)
 			, keyVariety = newvariety
-			, keySerialization = Nothing
 			}
 	{- Upgrade to fix bad previous migration that created a
 	 - non-extension preserving key, with an extension
@@ -198,7 +196,6 @@ trivialMigrate' oldkey newbackend afile maxextlen
 	| newvariety == oldvariety && not (hasExt oldvariety) &&
 		keyHash oldkey /= keyName oldkey = Just $ oldkey
 			{ keyName = keyHash oldkey
-			, keySerialization = Nothing
 			}
 	| otherwise = Nothing
   where
@@ -291,8 +288,5 @@ testKeyBackend =
 	let b = genBackendE (SHA2Hash (HashSize 256))
 	in b { getKey = (fmap addE) <$$> getKey b } 
   where
-	addE k = k
-		{ keyName = keyName k <> longext
-		, keySerialization = Nothing
-		}
+	addE k = k { keyName = keyName k <> longext }
 	longext = ".this-is-a-test-key"