Suppress error logs from `CopilotCompletionProvider` when Copilot is disabled (#9826)

This PR fixes some noisy error logs from the `CopilotCompletionProvider`
when Copilot is disabled entirely via the settings.

I have the following in my settings file:

```json
{
  "features": {
    "copilot": false
  },
}
```

After #9777 I started seeing my Zed logs getting filled up with messages
like this:

```
[2024-03-26T14:33:09-04:00 ERROR util] crates/copilot_ui/src/copilot_completion_provider.rs:206: copilot is disabled
```

Release Notes:

- N/A