Book: Update GHA doc and remove GitLab placeholder

Author: flip1995

book/src/SUMMARY.md

@@ -16,7 +16,6 @@
 - [Continuous Integration](continuous_integration/README.md)
     - [Travis CI](continuous_integration/travis.md)
     - [GitHub Actions](continuous_integration/github_actions.md)
-    - [Gitlab](continuous_integration/gitlab.md)
 - [Development](development/README.md)
     - [Basics](development/basics.md)
     - [Adding Lints](development/adding_lints.md)

book/src/continuous_integration/github_actions.md

@@ -1,9 +1,7 @@
 # GitHub Actions
 
-## actions-rs
-
-An easy way to get started with adding Clippy to GitHub Actions is
-the [actions-rs](https://github.com/actions-rs) [clippy-check](https://github.com/actions-rs/clippy-check):
+On the GitHub hosted runners, Clippy from the latest stable Rust version comes
+pre-installed. So all you have to do is to run `cargo clippy`.
 
 ```yml
 on: push