async keyword in RPC declaration

[pdeschain]

Describe the bug
It is possible to declare an RPC as async function, which is... weird. The fact that we can also call await within the RPC itself is also confusing.

To Reproduce
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2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
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Environment (please complete the following information):

• OS: [e.g. Windows 10]
• Unity Version: [e.g. 2019.1]
• MLAPI Version: [e.g. v6.0.1]
• MLAPI Commit: [e.g. https://github.com/Unity-Technologies/com.unity.multiplayer.mlapi/commit/c102935df1d7e0928283b48948fe96e5d96dd961]

Additional context
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[will-mearns]

Added to bug backlog for triaging, https://jira.unity3d.com/browse/MTTSDK-9