networkAgent: Do not remove child watch source

networkAgent: Do not remove child watch source

The child watch source is the only thing that reaps the child process.
We cannot remove it until it fires or the child will remain a zombie.
So, just leave the source attached. Trying to remove it does not seem
easy, and I don't see any need; we expect the child process to exit
quickly, so this doesn't seem harmful.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7083
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3926>
(cherry picked from commit 961a7d721f3e4db9be6df958b10ba98955138cae)