annex.backend is the new name for what was annex.backends

It takes a single key-value backend, rather than the unncessary and confusing list.
The old option still works if set.

Simplified some old old code too.

This commit was sponsored by Thomas Hochstein on Patreon.

Annex.hs

@@ -98,7 +98,7 @@ data AnnexState = AnnexState
 	{ repo :: Git.Repo
 	, repoadjustment :: (Git.Repo -> IO Git.Repo)
 	, gitconfig :: GitConfig
-	, backends :: [BackendA Annex]
+	, backend :: Maybe (BackendA Annex)
 	, remotes :: [Types.Remote.RemoteA Annex]
 	, remoteannexstate :: M.Map UUID AnnexState
 	, output :: MessageState
@@ -149,7 +149,7 @@ newState c r = do
 		{ repo = r
 		, repoadjustment = return
 		, gitconfig = c
-		, backends = []
+		, backend = Nothing
 		, remotes = []
 		, remoteannexstate = M.empty
 		, output = def