Rust: remove pre-1.94 format macro fallback

[paldepind]

Context

1. When we began the update to rust-analyzer 0.0.347 we noted an incompatibility with pre-1.94 Rust toolchain versions.
2. The commit 7faf81f put a workaround in place for this incompatibility: we we manually expand certain macros.
3. With Rust: Update rust-analyzer to 0.0.347 and pin toolchain #22493 we moved in a different direction, where we always do extraction with a 1.97.0 Rust toolchain.
4. The < 1.94 is now dead code.

This PR

The PR essentially just reverts 7faf81f, but it's not an exact revert, since some later commits tweaked things a bit.

Spelled out the PR:

• Deletes the synthetic reconstruction of format-family macro expansions
• Remove the dedicated pre-1.94 QL fixture and toolchain installation

[Copilot]

Copilot review overview

🟢 Approval recommended

The cleanup is internally consistent, preserves required environment handling, and includes appropriate validation.

Review tier: Balanced
Findings: None

What changed in this PR

Removes obsolete Rust toolchain overrides now that toolchain pinning is managed independently.

Changes:

• Removes fixed-toolchain selection, logging, and preinstallation.
• Uses standard cargo while preserving configured environment variables.
• Removes the direct ra_ap_toolchain dependency and regenerates Bazel metadata.

File	Description
rust/​ql/​test/​setup.sh	Stops preinstalling Rust 1.97.
rust/​ql/​integration-tests/​conftest.py	Removes integration-test toolchain installation.
rust/​extractor/​src/​toolchain.rs	Deletes fixed-toolchain logic.
rust/​extractor/​src/​qltest.rs	Runs standard cargo with configured environment.
rust/​extractor/​src/​main.rs	Removes toolchain logging and module registration.
rust/​extractor/​src/​config.rs	Stops overriding RUSTUP_TOOLCHAIN.
rust/​extractor/​Cargo.toml	Removes the direct toolchain dependency.
MODULE.bazel	Removes the obsolete direct Bazel repository import.
misc/​bazel/​3rdparty/​tree_sitter_extractors_deps/​ra_ap_toolchain/​BUILD.bazel	Deletes obsolete generated alias.
misc/​bazel/​3rdparty/​tree_sitter_extractors_deps/​ra_ap_toolchain-0.0.347/​BUILD.bazel	Deletes versioned generated alias.
misc/​bazel/​3rdparty/​tree_sitter_extractors_deps/​crates.bzl	Regenerates direct dependency metadata.
misc/​bazel/​3rdparty/​tree_sitter_extractors_deps/​BUILD.bazel	Removes top-level toolchain aliases.
Cargo.lock	Removes the extractor’s direct dependency entry.

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[hvitved]

The DCA run was lacking -X use-database-cache=false -X change-ql-submodule-in-semmle-code=true, so I have started a new run. Otherwise LGTM.

[paldepind]

Thanks for starting DCA correctly. There are some extraction errors. I'll investigate.

[paldepind]

Extraction errors are for DCA projects on an old nightly toolchain. For these projects the format-macro fallback is in fact not dead code.format-macro fallback is not dead code.

I'm putting back in draft to re-consider. We could go ahead as-is, but it might be better just to implement #22493 (comment) to avoid noise in QA.