chore: remove prefilter, stand-alone-test and default feature

[usamoi]

This pull request

• removes prefilter: the implementation is not usable at all and blocks future work
• removes stand-alone-test: it's just a copy of hnsw
• removes default feature pg15 in Cargo.toml to avoid too many --no-default-features for whom does not install PostgreSQL 15.

If you're developing with PostgreSQL 16, after this pr is merged, you need to add following json snippet to .vscode/settings.json.

"rust-analyzer.cargo.features": [
    "pg16"
],

[usamoi]

It does not add any new code, can it be merged (without waiting for 0.3.0 release)? @VoVAllen