Revert "core: treat ECHILD as child already terminated"

This reverts commit 268da271cc.

This breaks the test, but it is not clear that this is the
right fix. Revert for now, needs still investigation.

src/NetworkManagerUtils.c

@@ -566,7 +566,7 @@ nm_utils_kill_child_async (pid_t pid, int sig, guint64 log_domain,
 
 		/* let's try again with waitpid, probably there was a race... */
 		ret = waitpid (pid, &status, 0);
-		if (ret > 0 || (ret < 0 && errno == ECHILD)) {
+		if (ret > 0) {
 			nm_log_dbg (log_domain, LOG_NAME_FMT ": process %ld already terminated %s",
 			            LOG_NAME_ARGS, (long) ret, _kc_exit_to_string (buf_exit, status));
 			_kc_invoke_callback (pid, log_domain, log_name, callback, user_data, TRUE, status);