[#1025] ir: Read IR list from role management contract w/o mainchain

If mainchain is disabled in IR config then the node should read inner
ring list via role management contract.

Use `NeoFSAlphabetList` method of morph client as IR lister if
`withoutMainNet` flag is set in IR app.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/innerring/innerring.go

@@ -536,7 +536,9 @@ func New(ctx context.Context, log *zap.Logger, cfg *viper.Viper) (*Server, error
 
 	var irf irFetcher
 
-	if !server.mainNotaryConfig.disabled {
+	if server.withoutMainNet || !server.mainNotaryConfig.disabled {
+		// if mainchain is disabled we should use NeoFSAlphabetList client method according to its docs
+		// (naming `...WithNotary` will not always be correct)
 		irf = NewIRFetcherWithNotary(server.morphClient)
 	} else {
 		irf = NewIRFetcherWithoutNotary(server.netmapClient)