Signing context cleanup (#2568)

* Signing Context cleanup

Removes extra NBXSeedAvailable property in SigningContext, which got introduced in #2559. It is not needed since each view model involved already has it.

* Fix button in link

BTCPayServer/Controllers/WalletsController.PSBT.cs

@@ -119,7 +119,6 @@ namespace BTCPayServer.Controllers
             }
 
             vm.PSBTHex = psbt.ToHex();
-            vm.SigningContext.NBXSeedAvailable = vm.NBXSeedAvailable;
             var routeBack = new Dictionary<string, string>
             {
                 {"action", nameof(WalletPSBT)}, {"walletId", walletId.ToString()}