Fix scripts/bench-branches.py

[kmcallister]

It uses the removed libtest --save-metrics feature. See 22a97c2 and rust-lang/rust#19145.

[Ygg01]

We removed this one with #268 I'm assuming it's no longer relevant.

[SimonSapin]

Let’s close, it’s in git history if we want to fix it and use it again later.