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.

Utility/Hash.hs

@@ -27,7 +27,6 @@ module Utility.Hash (
 	blake2b_512,
 #endif
 	md5,
-	md5s,
 	prop_hashes_stable,
 	Mac(..),
 	calcMac,
@@ -113,9 +112,6 @@ blake2b_512 = hashlazy
 md5 ::  L.ByteString -> Digest MD5
 md5 = hashlazy
 
-md5s ::  S.ByteString -> Digest MD5
-md5s = hash
-
 {- Check that all the hashes continue to hash the same. -}
 prop_hashes_stable :: Bool
 prop_hashes_stable = all (\(hasher, result) -> hasher foo == result)