[RustSDK] Remove tx subscription example in the Rust SDK (#20610)

## Description 

Let's remove the example - first it is fairly basic, and second, WS is
off on most nodes so people cannot run this example anymore.

Should close #20607

## Test plan 

How did you test the new or updated feature?

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## Release notes

Check each box that your changes affect. If none of the boxes relate to
your changes, release notes aren't required.

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that describes the impact of your changes that a user might notice and
any actions they must take to implement updates.

- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] Indexer: 
- [ ] JSON-RPC: 
- [ ] GraphQL: 
- [ ] CLI: 
- [ ] Rust SDK:
- [ ] REST API: