doc: remove superfluous text in onboarding-extras

PR-URL: #19247
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/onboarding-extras.md

@@ -50,9 +50,7 @@ If you cannot find who to cc for a file, `git shortlog -n -s <file>` may help.
 
 ## Labels
 
-### By Subsystem
-
-Subsystems include:
+### Subsystems
 
 * `lib/*.js` (`assert`, `buffer`, etc.)
 * `build`
@@ -66,8 +64,6 @@ request.
 
 ### General
 
-Please use these when possible / appropriate
-
 * `confirmed-bug` - Bugs you have verified exist
 * `discuss` - Things that need larger discussion
 * `feature request` - Any issue that requests a new feature (usually not PRs)