Don't check asynchronous cancellation on system

Since asynchronous cancellation was removed from system by

commit c4dd57c300aa05ab2f957d571c321237a58d0368
Author: Ondřej Bílka <neleai@seznam.cz>
Date:   Tue Jan 14 16:07:50 2014 +0100

    Do not enable asynchronous cancellation in system. Fixes bug 14782.

    We needlessly enabled thread cancellation before it was necessary.
As
    only call that needs to be guarded is waitpid which is cancellation
    point we could remove cancellation altogether.

we shouldn't check asynchronous cancellation on system.

	[BZ #14782]
	* tst-cancel-wrappers.sh: Remove system.