landlock: Remove KUnit test that triggers a warning

A KUnit test checking boundaries triggers a canary warning, which may be
disturbing.  Let's remove this test for now.  Hopefully, KUnit will soon
get support for suppressing warning backtraces [1].

Cc: Alessandro Carminati <acarmina@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Günther Noack <gnoack@google.com>
Reported-by: Tingmao Wang <m@maowtm.org>
Closes: https://lore.kernel.org/r/20250327213807.12964-1-m@maowtm.org
Link: https://lore.kernel.org/r/20250425193249.78b45d2589575c15f483c3d8@linux-foundation.org [1]
Link: https://lore.kernel.org/r/20250503065359.3625407-1-mic@digikod.net


Signed-off-by: Mickaël Salaün <mic@digikod.net>