remove Preparer abstraction

That had almost no benefit at all, and complicated things quite a lot.

What I proably wanted this to be was something like ResourceT, but it
was not. The few remotes that actually need some preparation done only
once and reused used a MVar and not Preparer.

Remote/GCrypt.hs

@@ -159,10 +159,10 @@ gen' r u c gc rs = do
 		, remoteStateHandle = rs
 	}
 	return $ Just $ specialRemote' specialcfg c
-		(simplyPrepare $ store this rsyncopts)
-		(simplyPrepare $ retrieve this rsyncopts)
-		(simplyPrepare $ remove this rsyncopts)
-		(simplyPrepare $ checkKey this rsyncopts)
+		(store this rsyncopts)
+		(retrieve this rsyncopts)
+		(remove this rsyncopts)
+		(checkKey this rsyncopts)
 		this
   where
 	specialcfg