Remove code owners for build files (#4844)

b/377271824

Remove cobalt-build-owners check for *gn* files.
Keep the check for gn.py

.github/CODEOWNERS

@@ -2,14 +2,9 @@
 
 /media/ @youtube/cobalt-media
 
-# Consider removing the gn, gni files code owners in April 2025 once
-# people are more familiar with Cobalt-Chrome GN changes.
-*.gn @youtube/cobalt-build
-*.gni @youtube/cobalt-build
-
 /.github/  # no owners
 /cobalt/  # no owners
 /starboard/  # no owners
 /components/viz/service/display/starboard/  # no owners
 /ui/ozone/platform/starboard/  # no owners
-/cobalt/build @youtube/cobalt-build
+/cobalt/build/gn.py @youtube/cobalt-build