Remove automatic feature freeze comment (#15690)

The feature freeze period is over.

changelog: none

Author: blyxyas

.github/workflows/feature_freeze.yml

@@ -1,9 +1,5 @@
 # Clippy
 
-[### IMPORTANT NOTE FOR CONTRIBUTORS ================](development/feature_freeze.md)
-
-----
-
 [![License: MIT OR Apache-2.0](https://img.shields.io/crates/l/clippy.svg)](https://github.com/rust-lang/rust-clippy#license)
 
 A collection of lints to catch common mistakes and improve your

book/src/README.md

@@ -13,7 +13,6 @@
     - [GitLab CI](continuous_integration/gitlab.md)
     - [Travis CI](continuous_integration/travis.md)
 - [Development](development/README.md)
-    - [IMPORTANT: FEATURE FREEZE](development/feature_freeze.md)
     - [Basics](development/basics.md)
     - [Adding Lints](development/adding_lints.md)
     - [Defining Lints](development/defining_lints.md)

book/src/SUMMARY.md

@@ -1,8 +1,5 @@
 # Adding a new lint
 
-[### IMPORTANT NOTE FOR CONTRIBUTORS ================](feature_freeze.md)
-
-
 You are probably here because you want to add a new lint to Clippy. If this is
 the first time you're contributing to Clippy, this document guides you through
 creating an example lint from scratch.