rt: rm internal Park,Unpark traits (#4991)

Years ago, Tokio was organized as a cluster of separate crates. This
architecture required a trait to represent "park the thread and do
work.". When all crates were combined into a single crate, the `Park`
and `Unpark` traits remained as internal details.

This patch removes these traits as they are no longer needed. This is in
service of a future refactor that will decouple the various resource
drivers and store them in a single struct instead of nesting them.
However, in the mean time, removing the Park/Unpark traits adds a bit of
messy code to make the conditional compilation work. This code will
(hopefully) be short-lived.

.github/workflows/ci.yml

@@ -281,6 +281,8 @@ jobs:
         run: cargo install cargo-hack
       - name: check --each-feature
         run: cargo hack check --all --each-feature -Z avoid-dev-deps
+      - name: check net,time
+        run: cargo check -p tokio --no-default-features --features net,time -Z avoid-dev-deps
       # Try with unstable feature flags
       - name: check --each-feature --unstable
         run: cargo hack check --all --each-feature -Z avoid-dev-deps