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/Models/WalletViewModels/SigningContextModel.cs

@@ -17,6 +17,5 @@ namespace BTCPayServer.Models.WalletViewModels
         public string PayJoinBIP21 { get; set; }
         public bool? EnforceLowR { get; set; }
         public string ChangeAddress { get; set; }
-        public bool NBXSeedAvailable { get; set; }
     }
 }