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.

Command/AddUrl.hs

@@ -377,10 +377,7 @@ finishDownloadWith tmp u url file = do
 
 {- Adds the url size to the Key. -}
 addSizeUrlKey :: Url.UrlInfo -> Key -> Key
-addSizeUrlKey urlinfo key = key
-	{ keySize = Url.urlSize urlinfo
-	, keySerialization = Nothing
-	}
+addSizeUrlKey urlinfo key = key { keySize = Url.urlSize urlinfo }
 
 {- Adds worktree file to the repository. -}
 addWorkTree :: UUID -> URLString -> FilePath -> Key -> Maybe FilePath -> Annex ()