Remove example of manually setting RUSTFLAGS

Author: Kobzol

src/doc/src/guide/build-performance.md

@@ -71,16 +71,9 @@ Trade-offs:
 
 ### Enable the experimental parallel frontend
 
-Recommendation: Add `-Zthreads=n` to the `RUSTFLAGS` environment variable, for example:
-
-```console
-$ RUSTFLAGS="-Zthreads=8" cargo +nightly build
-```
-
-You can configure `RUSTFLAGS` using [Cargo config](../reference/config.md#buildrustflags), so that you do not have to pass the flag on every cargo invocation manually:
+Recommendation: Add `-Zthreads=n` to the `RUSTFLAGS` environment variable, for example by adding the following to `.cargo/config.toml`:
 
 ```toml
-# .cargo/config.toml
 [build]
 rustflags = "-Zthreads=8"
 ```
@@ -93,3 +86,4 @@ Trade-offs:
 
 [parallel-frontend-blog]: https://blog.rust-lang.org/2023/11/09/parallel-rustc/
 [parallel-frontend-issue]: https://github.com/rust-lang/rust/issues/113349
+