[#1514] adm/nns: Do not return hash from getRPCClient()

It was unused and we employ better abstractions now.
gopatch:
```
@@
var a, b expression
@@
-a, b, _ := getRPCClient(...)
+a, b := getRPCClient(...)
```

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

cmd/frostfs-adm/internal/modules/morph/nns/renew.go

@@ -14,7 +14,7 @@ func initRenewCmd() {
 }
 
 func renewDomain(cmd *cobra.Command, _ []string) {
-	c, actor, _ := getRPCClient(cmd)
+	c, actor := getRPCClient(cmd)
 	name, _ := cmd.Flags().GetString(nnsNameFlag)
 	h, vub, err := c.Renew(name)
 	commonCmd.ExitOnErr(cmd, "unable to renew domain: %w", err)