native: don't register standby validators on initialization

C# doesn't do that since neo-project/neo#1762.

pkg/rpc/server/server_test.go

@@ -451,6 +451,7 @@ var rpcTestCases = map[string][]rpcTestCase{
 			result: func(*executor) interface{} {
 				return &[]result.Validator{}
 			},
+			/* preview3 doesn't return any validators until there is a vote
 			check: func(t *testing.T, e *executor, validators interface{}) {
 				var expected []result.Validator
 				sBValidators := e.chain.GetStandByValidators()
@@ -467,6 +468,7 @@ var rpcTestCases = map[string][]rpcTestCase{
 
 				assert.ElementsMatch(t, expected, *actual)
 			},
+			*/
 		},
 	},
 	"getversion": {