chore: remove build-time tokio-unstable cfg check

This is incompatible with envs that set RUSTFLAGS (by design), however
this is a bit of a over step - people should be able to configure their
env!

tokio_metrics_bridge/build.rs

@@ -1,14 +0,0 @@
-fn main() -> Result<(), Box<dyn std::error::Error>> {
-    let tokio_unstable = std::env::var("CARGO_CFG_TOKIO_UNSTABLE").is_ok();
-    if !tokio_unstable {
-        return Err("\
-RUSTFLAGS got overwritten -- potentially by your environment -- and does not \
-contain `--cfg tokio_unstable` anymore. Don't do that. If you want to adjust \
-build configs, either edit the repo-level `Cargo.toml` or the repo-level \
-`.cargo/config`. Ensure that it contains `--cfg tokio_unstable`."
-            .to_owned()
-            .into());
-    }
-
-    Ok(())
-}