docs: remove outdated pre-commit instructions (testing-library#909)

Co-authored-by: Sebastian Silbermann <[email protected]>

CONTRIBUTING.md

@@ -32,18 +32,6 @@ Please make sure to run the tests before you commit your changes. You can run
 `npm run test:update` which will update any snapshots that need updating. Make
 sure to include those changes (if they exist) in your commit.
 
-### opt into git hooks
-
-There are git hooks set up with this project that are automatically installed
-when you install dependencies. They're really handy, but are turned off by
-default (so as to not hinder new contributors). You can opt into these by
-creating a file called `.opt-in` at the root of the project and putting this
-inside:
-
-```
-pre-commit
-```
-
 ## Help needed
 
 Please checkout the [the open issues][issues]