Remove input labels from the LabelNames API response

#### Is your feature request related to a problem? Please describe.

When calling the `LabelNames` API with label selectors, the passed labels are returned in the response. E.g., if we pass a label selector such as `{container="ingester", pod="ingester-1"}`, the response will contain `container` and `pod` as label names (among other results). Since they are already selected, they don't serve a purpose and the existing frontends have logic to filter them out.

#### Describe the solution you'd like

We should remove the request labels from the response, as they are never used.

#### Additional context

https://github.com/grafana/grafana/pull/78861#discussion_r1409922759