opt-dist: don't set RUST_LOG=collector=debug

see [Zulip
discussion](https://rust-lang.zulipchat.com/#narrow/channel/122651-general/topic/opt-dist.3A.20do.20not.20set.20RUST_LOG.3Dcollector.3Ddebug.20forcefully)

Author: ognevny

src/tools/opt-dist/src/training.rs

@@ -39,7 +39,6 @@ fn init_compiler_benchmarks(
         "--exact-match",
         crates.join(",").as_str(),
     ])
-    .env("RUST_LOG", "collector=debug")
     .env("RUSTC", env.rustc_stage_0().as_str())
     .env("RUSTC_BOOTSTRAP", "1")
     .workdir(&env.rustc_perf_dir());