Remove legacy Tree-sitter symbol extractor plugin (plugins/symbol_extractor_treesitter),
the unused plugin-symbols build option, and associated tests and benchmark targets:
- Drop plugins/symbol_extractor_treesitter/ (grammar_loader, symbol_extractor, plugin_api).
- Remove plugin-symbols Meson option and packaging references.
- Remove dedicated symbol plugin tests and benchmark targets (symbol_extraction_bench,
symbol_extractor_all_langs_catch2_test, symbol_extractor_edge_cases_catch2_test,
symbol_extractor_solidity_catch2_test, symbol_extractor_query_test,
symbol_extractor_validation_test).
- Clean up dead symbol extraction plugin helper and test from hnsw_entity_search_bench.
- Prune obsolete portability allowlist entries.
Signed-off-by: trvon <me@trevon.dev>
