Update PR template

We decided to keep the PR template concise.

.github/PULL_REQUEST_TEMPLATE.md

@@ -17,14 +17,6 @@ and code of conduct https://github.com/grafana/k6/blob/master/CODE_OF_CONDUCT.md
 - [ ] I have added tests for my changes
 - [ ] I have commented on my code, particularly in hard-to-understand areas
 
-<!--
-If relevant:
-- [ ] I have updated the k6 release notes: PR link
-- [ ] I have updated the k6 documentation: PR link
-- [ ] I have updated [k6 browser get started](https://k6.io/blog/get-started-with-k6-browser/) blog: PR link
-- [ ] Generated and updated the [TypeScript definitions](https://github.com/DefinitelyTyped/DefinitelyTyped/tree/master/types/k6/experimental/browser)
--->
-
 ## Related PR(s)/Issue(s)
 
 <!-- Does it close an issue? -->