Discovery does not return token_endpoint_auth_signing_alg_values_supported

`token_endpoint_auth_signing_alg_values_supported` is only related to
private_key_jwt and client_secret_jwt client authentication methods
at the token endpoint, which we do not support. See
https://openid.net/specs/openid-connect-discovery-1_0.html#ProviderMetadata
for more details.

Signed-off-by: Aram Price <pricear@vmware.com>

test/integration/supervisor_discovery_test.go

@@ -472,7 +472,6 @@ func requireWellKnownEndpointIsWorking(t *testing.T, supervisorScheme, superviso
       "authorization_endpoint": "%s/oauth2/authorize",
       "token_endpoint": "%s/oauth2/token",
       "token_endpoint_auth_methods_supported": ["client_secret_basic"],
-      "token_endpoint_auth_signing_alg_values_supported": ["RS256"],
       "jwks_uri": "%s/jwks.json",
       "scopes_supported": ["openid", "offline"],
       "response_types_supported": ["code"],