Docs: Remove the "Grunt build" section from the PR template

Now that unit tests are run on GitHub Actions in all three major
engines and for multiple custom jQuery builds, the request for PR
authors to run unit tests locally and confirm they pass is needless
overhead; let's drop the checkbox.

Closes gh-5261

(cherry picked from commit 988a568)

Author: mgol

.github/PULL_REQUEST_TEMPLATE.md

@@ -11,7 +11,6 @@ Mark an `[x]` for completed items, if you're not sure leave them unchecked and w
 -->
 
 * [ ] New tests have been added to show the fix or feature works
-* [ ] Grunt build and unit tests pass locally with these changes
 * [ ] If needed, a docs issue/PR was created at https://github.com/jquery/api.jquery.com
 
 <!--