Revert "selftests/timens: add a test for vfork+exit"

The next patch reverts the code that this test verified.

This reverts commit 6342140d

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Signed-off-by: Andrei Vagin <avagin@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220913102551.1121611-2-avagin@google.com