KVM: arm64: selftests: Avoid testing the IMPDEF behavior

KVM: arm64: selftests: Avoid testing the IMPDEF behavior

It turned out that we can't really force KVM to use the "slow" path when
emulating AT instructions [1]. We should therefore avoid testing the IMPDEF
behavior (i.e., TEST_ACCESS_FLAG - address translation instructions are
permitted to update AF but not required).

Remove it and improve the comment a bit.

[1] https://lore.kernel.org/r/b951dcfb-0ad1-4d7b-b6ce-d54b272dd9be@linux.dev

Signed-off-by: Zenghui Yu (Huawei) <zenghui.yu@linux.dev>
Link: https://patch.msgid.link/20260317131558.52751-1-zenghui.yu@linux.dev
Signed-off-by: Marc Zyngier <maz@kernel.org>