Revert "core/service: when resetting PID also reset known flag"

This reverts commit ff32060.

This change is incorrect as we don't want to mark the PID as invalid but
only mark it as dead.

The change in question also breaks user level socket activation for
`podman.service` as the termination of the main `podman system service`
process is not properly handled, causing any application accessing the
socket to hang.

This is because the user-level `podman.service` unit also hosts two
non-main processes: `rootlessport` and `rootlessport-child` which causes
the `cgroup_good` check to still succeed.

The original submitter of this commit is recommended to find another
more correct way to fix the cgroupsv1 issue on CentOS 8.

(cherry picked from commit f29f087)

src/core/service.c

@@ -3529,7 +3529,6 @@ static void service_sigchld_event(Unit *u, pid_t pid, int code, int status) {
                         return;
 
                 s->main_pid = 0;
-                s->main_pid_known = false;
                 exec_status_exit(&s->main_exec_status, &s->exec_context, pid, code, status);
 
                 if (s->main_command) {