Removes unstable feature flag from CI doc commands

Per Stjepan Glavina, this flag is superfluous because #[cfg(any(feature = "unstable", feature = "docs"))]
is used everywhere in the codebase.

Discussion: https://github.com/async-rs/async-std/pull/278#issuecomment-538744737

.github/workflows/ci.yml

@@ -54,4 +54,4 @@ jobs:
       run: cargo fmt --all -- --check
 
     - name: Docs
-      run: cargo doc --features docs,unstable
+      run: cargo doc --features docs

.travis.yml

@@ -50,7 +50,7 @@ matrix:
       rust: nightly
       os: linux
       script:
-        - cargo doc --features docs,unstable
+        - cargo doc --features docs
 
     - name: book
       rust: nightly