simplify base64 to only use ByteString

Note the use of fromString and toString from Data.ByteString.UTF8 dated
back to commit 9b93278e8a. Back then it
was using the dataenc package for base64, which operated on Word8 and
String. But with the switch to sandi, it uses ByteString, and indeed
fromB64' and toB64' were already using ByteString without that
complication. So I think there is no risk of such an encoding related
breakage.

I also tested the case that 9b93278e8a
fixed:

	git-annex metadata -s foo='a …' x
	git-annex metadata x
	metadata x
	  foo=a …

In Remote.Helper.Encryptable, it was avoiding using Utility.Base64
because of that UTF8 conversion. Since that's no longer done, it can
just use it now.

Utility/Base64.hs

@@ -1,55 +1,25 @@
 {- Simple Base64 encoding
  -
- - Copyright 2011-2019 Joey Hess <id@joeyh.name>
+ - Copyright 2011-2023 Joey Hess <id@joeyh.name>
  -
  - License: BSD-2-clause
  -}
 
-{-# LANGUAGE PackageImports #-}
-
 module Utility.Base64 where
 
-import Utility.FileSystemEncoding
-import Utility.QuickCheck
 import Utility.Exception
 
-import qualified "sandi" Codec.Binary.Base64 as B64
+import Codec.Binary.Base64 as B64
 import Data.Maybe
 import qualified Data.ByteString as B
-import Data.ByteString.UTF8 (fromString, toString)
-import Data.Char
 
--- | This uses the FileSystemEncoding, so it can be used on Strings
--- that represent filepaths containing arbitrarily encoded characters.
-toB64 :: String -> String
-toB64 = toString . B64.encode . encodeBS
+toB64 :: B.ByteString -> B.ByteString
+toB64 = B64.encode
 
-toB64' :: B.ByteString -> B.ByteString
-toB64' = B64.encode
+fromB64Maybe :: B.ByteString -> Maybe (B.ByteString)
+fromB64Maybe = either (const Nothing) Just . B64.decode
 
-fromB64Maybe :: String -> Maybe String
-fromB64Maybe s = either (const Nothing) (Just . decodeBS)
-	(B64.decode $ fromString s)
-
-fromB64Maybe' :: B.ByteString -> Maybe (B.ByteString)
-fromB64Maybe' = either (const Nothing) Just . B64.decode
-
-fromB64 :: String -> String
+fromB64 :: B.ByteString -> B.ByteString
 fromB64 = fromMaybe bad . fromB64Maybe
   where
 	bad = giveup "bad base64 encoded data"
-
-fromB64' :: B.ByteString -> B.ByteString
-fromB64' = fromMaybe bad . fromB64Maybe'
-  where
-	bad = giveup "bad base64 encoded data"
-
--- Only ascii strings are tested, because an arbitrary string may contain
--- characters not encoded using the FileSystemEncoding, which would thus
--- not roundtrip, as decodeBS always generates an output encoded that way.
-prop_b64_roundtrips :: TestableString -> Bool
-prop_b64_roundtrips ts
-	| all (isAscii) s = s == decodeBS (fromB64' (toB64' (encodeBS s)))
-	| otherwise = True
-  where
-	s = fromTestableString ts