Remove incorrect signing key setter

- Those are never used, as the signature is validated separately.
- Those keys are the SP signing keys, not the Idp signing keys.

Sustainsys.Saml2/SAML2P/Saml2Response.cs

@@ -555,8 +555,6 @@ private IEnumerable<ClaimsIdentity> CreateClaims(IOptions options)
             validationParameters.ValidAudience = options.SPOptions.EntityId.Id;
 
 			var handler = options.SPOptions.Saml2PSecurityTokenHandler;
-			validationParameters.IssuerSigningKeys = options.SPOptions
-				.ServiceCertificates.Select(x => new X509SecurityKey(x.Certificate));
 
 			foreach (XmlElement assertionNode in GetAllAssertionElementNodes(options))
             {