dotnet-suggest register --help lists --suggestion-command, which has no effect

The output of `dotnet-suggest register --help` describes this option:

```
--suggestion-command The command to invoke to retrieve suggestions
```

However, the option has no effect. The feature was removed in . Please remove it from the `--help` output as well, or change its description to state that it has no effect.

Search the register command's help text for `--suggestion-command`, then review the related feature-removal commit mentioned in the issue. Remove the option from the help output or update its description to explain that it has no effect, and verify that `dotnet-suggest register --help` no longer misleadingly documents it.

Written by the indexing model from the issue text.