Fix misguided suggestions in config.toml

[jyn514]

• verbose-tests is for debugging the test harness, not the tests
  themselves. See also Address review comments about config.toml from rustc-dev-guide PR rust#76141
• codegen-units defaults to 256 whenever incremental = true. So
  there's no need to explicitly set it to 0 if we already recommend
  incremental. See also Resync config.toml and suggest better settings #795 (comment).

r? @eddyb

[jyn514]

error: Server returned 404 Not Found for https://github.com/rust-lang/rust/tree/master/src/librustc_metadata

    ┌─ query.md:155:1

    │

155 │ [`rustc_metadata` crate][rustc_metadata], which loads the information

    │ ^ Server returned 404 Not Found for https://github.com/rust-lang/rust/tree/master/src/librustc_metadata

Looks like this needs the changes from #838 before CI will pass.