doc: clarify issues and pull requests guidance

Removed the "Managing Issues and Pull Requests" header as "managing" is
unclear in this context.

Useful information from the section was retained as an introductory
paragraph for the "Issues and Pull Requests" section. Vague terminology
(e.g., "full responsibility") was removed.

PR-URL: #24316
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

COLLABORATOR_GUIDE.md

@@ -3,7 +3,6 @@
 ## Contents
 
 * [Issues and Pull Requests](#issues-and-pull-requests)
-  - [Managing Issues and Pull Requests](#managing-issues-and-pull-requests)
   - [Welcoming First-Time Contributors](#welcoming-first-time-contributors)
   - [Closing Issues and Pull Requests](#closing-issues-and-pull-requests)
   - [Author ready pull requests](#author-ready-pull-requests)
@@ -45,14 +44,9 @@ Collaborators should understand the
 
 ## Issues and Pull Requests
 
-### Managing Issues and Pull Requests
-
-Collaborators should take full responsibility for managing issues and pull
-requests they feel qualified to handle. Make sure this is done while being
-mindful of these guidelines, the opinions of other Collaborators, and guidance
-of the [TSC][]. They may also notify other qualified parties for more input on
-an issue or a pull request.
-See [Who to CC in the issue tracker](#who-to-cc-in-the-issue-tracker).
+Mind these guidelines, the opinions of other Collaborators, and guidance of the
+[TSC][]. Notify other qualified parties for more input on an issue or a pull
+request. See [Who to CC in the issue tracker](#who-to-cc-in-the-issue-tracker).
 
 ### Welcoming First-Time Contributors