chore: delete unused file (#2280)

The `blocking` module is not referenced in code. The file is left over
from an earlier refactor.

tokio/src/blocking/mod.rs

@@ -1,11 +0,0 @@
-#![cfg_attr(not(feature = "blocking"), allow(dead_code, unused_imports))]
-
-//! Perform blocking operations from an asynchronous context.
-
-cfg_blocking_impl! {
-    mod pool;
-    pub(crate) use pool::{spawn_blocking, BlockingPool, Spawner};
-
-    mod schedule;
-    mod task;
-}