Inspect code smell

[fulmicoton]

Inspect whether the merge_enabled argument in this function makes snese.

   async fn spawn_merge_pipeline(
        &mut self,
        ctx: &ActorContext<Self>,
        index_id: String,
        merge_enabled: bool,
    ) -> Result<IndexingPipelineId, IndexingServiceError> {
        let pipeline_id = IndexingPipelineId {
            index_id: index_id.clone(),
            source_id: "void-source".to_string(),
            node_id: self.node_id.clone(),
            pipeline_ord: 0,