build: don't call cargo in build.rs

This doesn't work when building outside of Cargo (e.g. using Bazel that
invoke rustc directly).

build.rs

@@ -9,9 +9,4 @@ fn main() {
             .unwrap_or_default(),
         None => println!("no need for pb"),
     }
-
-    Command::new("cargo")
-        .args(["fmt", "--", "src/*.rs"])
-        .status()
-        .expect("cargo fmt failed");
 }