Removing has_fts field from split-duckdb-index
Context: https://github.com/huggingface/dataset-viewer/pull/2928#discussion_r1652733919

I'm removing the `has_fts` field from `split-duckdb-index`, given that now, the `stemmer` field will indicate if the split supports the feature. `stemmer`=None means no FTS; otherwise, it does.

Removing has_fts field from split-duckdb-index: Context: https://github.com/huggingface/dataset-viewer/pull/2928#discussion_r1652733919

I'm removing the `has_fts` field from `split-duckdb-index`, given that now, the `stemmer` field will indicate if the split supports the feature. `stemmer`=None means no FTS; otherwise, it does.
