darwinvlc: remove SIGCHLD GCD handling

The handling through GCD was added back then in commit cc07bce0. In that commit, signal(SIGCHLD, SIG_DFL) was already called to ensure the behavior of the signal is restored to default handlers.

In man 3 signal from the macos manual, it is said that when setting SIG_IGN for SIGCHLD, the operating system will not create zombie process and the exit status will be ignored. We do want to avoid creating zombie process but we also want to have exit status, which means we do indeed need the SIG_DFL handler here. See also the following commit on the linux side: 94763831.

However, GCD was used to automatically call waitpid() on the process that are being signalled as closing. Since we're always calling waitpid when spawning any process, this duplicates the handling. But the clients are actually using vlc_waitpid which is checking errno != ECHILD, so it would break as soon as the GCD handler is executed before the vlc_waitpid call.

Since every cases are explicitely calling vlc_waitpid(), do as linux and just remove the specific GCD handling.

Co-authored-by: Marvin Scholz epirat07@gmail.com

Fixes #29099