refactor(python): remove legacy tantivy FTS support (#3282)

This follows the Rust-side Tantivy removal by deleting the remaining
Python Tantivy runtime, tests, and packaging references.

It also turns the legacy Python-only Tantivy parameters into explicit
errors and stops reading legacy `_indices/fts` directories so Python FTS
is fully native-only.

python/python/tests/test_db.py

@@ -15,8 +15,7 @@ import pytest
 from lancedb.pydantic import LanceModel, Vector
 
 
-@pytest.mark.parametrize("use_tantivy", [True, False])
-def test_basic(tmp_path, use_tantivy):
+def test_basic(tmp_path):
     db = lancedb.connect(tmp_path)
 
     assert db.uri == str(tmp_path)
@@ -49,7 +48,7 @@ def test_basic(tmp_path, use_tantivy):
     assert len(rs) == 1
     assert rs["item"].iloc[0] == "foo"
 
-    table.create_fts_index("item", use_tantivy=use_tantivy)
+    table.create_fts_index("item")
     rs = table.search("bar", query_type="fts").to_pandas()
     assert len(rs) == 1
     assert rs["item"].iloc[0] == "bar"