Do not prefill https:// in prompt for Databricks Host (#1364)

## Changes
This PR is a minor UX improvement. By not autofilling the https://
prefix in Databricks Host we allow users to directly copy-paste from
their browser.

UX:
```
➜  cli git:(fix/copy-host) cli auth login
Databricks Profile Name: my-profile
Databricks Host (e.g. https://<databricks-instance>.cloud.databricks.com): https://foobar.cloud.databricks.com
Profile my-profile was successfully saved
```

## Tests
Manually.

cmd/auth/auth.go

@@ -35,9 +35,7 @@ GCP: https://docs.gcp.databricks.com/en/dev-tools/auth/index.html`,
 
 func promptForHost(ctx context.Context) (string, error) {
 	prompt := cmdio.Prompt(ctx)
-	prompt.Label = "Databricks Host"
-	prompt.Default = "https://"
-	prompt.AllowEdit = true
+	prompt.Label = "Databricks Host (e.g. https://<databricks-instance>.cloud.databricks.com)"
 	// Validate?
 	host, err := prompt.Run()
 	if err != nil {