Issue 503: ServerSceneChanged event invoked twice on server start

Repository: miragenet/mirage

Issue body:
**Describe the bug**
ServerSceneChanged event on NetworkSceneManager gets invoked twice when starting the server.

**Repro project**
Add a Game object to Basic example with the [following script](https://hastebin.com/ecixafalil.csharp).

**To Reproduce**
Steps to reproduce the behavior:
1. Open Basic example.
2. Add empty game object.
3. App [CallbackTest](https://hastebin.com/ecixafalil.csharp).
4. Enter playmode

**Expected behavior**
ServerSceneChanged gets fired once.

**Actual behavior**
ServerSceneChanged gets fired twice.

**Desktop**
* OS: Windows 10
* Build target: Standalone x64
* Unity version: 2020.1
* Mirror branch: 59.2.0
