Re-enable tests disabled by issue 10

PR dotnet#1502 fixed issue dotnet#10 by implementing UpnEndpointIdentity.
But 2 tests were still disabled by issue 10, so this PR just
re-enables those tests.  Both require manual setup to run, so
we will not see them pass or fail in CI or VSO.

src/System.Private.ServiceModel/tests/Scenarios/Security/TransportSecurity/Negotiate/NegotiateStream_Http_Tests.4.1.0.cs

@@ -260,7 +260,6 @@ public static void NegotiateStream_Http_With_ExplicitSpn()
     [Condition(nameof(Windows_Authentication_Available),
                nameof(Root_Certificate_Installed),
                nameof(UPN_Available))]
-    [Issue(10)]
     [OuterLoop]
     public static void NegotiateStream_Http_With_Upn()
     {
@@ -406,7 +405,6 @@ public static void NegotiateStream_Http_With_ExplicitUserNameAndPassword_With_Sp
                nameof(Explicit_Credentials_Available),
                nameof(Domain_Available),
                nameof(UPN_Available))]
-    [Issue(10)]
     [OuterLoop]
     public static void NegotiateStream_Http_With_ExplicitUserNameAndPassword_With_Upn()
     {