[XCM]: Remove NetworkIds for testnets (#108)

Testnets are not stable and shouldn't be included in the XCM language.
They should be able to be specified with `NetworkId::ByGenesis`.

text/0108-xcm-remove-testnet-ids.md

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+# RFC-0108: Remove XCM testnet NetworkIds
+
+|                 |                                                                                             |
+| --------------- | ------------------------------------------------------------------------------------------- |
+| **Start Date**  | 23 July 2024                                                                                |
+| **Description** | Remove the NetworkIds for testnets Westend and Rococo                                       |
+| **Authors**     |                                                                                             |
+
+## Summary
+
+This RFC aims to remove the `NetworkId`s of `Westend` and `Rococo`, arguing that testnets shouldn't go in the language.
+
+## Motivation
+
+We've already seen the plans to phase out Rococo and Paseo has appeared.
+Instead of constantly changing the testnets included in the language, we should favor specifying them via their genesis hash,
+using `NetworkId::ByGenesis`.
+
+## Stakeholders
+
+- Runtime devs
+- Wallets
+- dApps
+
+## Explanation
+
+Remove `Westend` and `Rococo` from the included `NetworkId`s in the language.
+
+## Drawbacks
+
+This RFC will make it less convenient to specify a testnet, but not by a large amount.
+
+## Testing, Security, and Privacy
+
+None.
+
+## Performance, Ergonomics, and Compatibility
+
+### Performance
+
+None.
+
+### Ergonomics
+
+It will very slightly reduce the ergonomics of testnet developers but improve the stability of the language.
+
+### Compatibility
+
+`NetworkId::Rococo` and `NetworkId::Westend` can just use `NetworkId::ByGenesis`, as can other testnets.
+
+## Prior Art and References
+
+A previous attempt to add `NetworkId::Paseo`: https://github.com/polkadot-fellows/xcm-format/pull/58.
+
+## Unresolved Questions
+
+None.
+
+## Future Directions and Related Material
+
+None.