KVM: arm64: Remove duplicated AA64MMFR1_EL1 XNX

Commit d5a32b60 ("KVM: arm64: Allow userspace to change
ID_AA64MMFR{0-2}_EL1") made certain fields in these registers writable,
but in doing so, ID_AA64MMFR1_EL1_XNX was listed twice. Remove the
duplication.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Zenghui Yu <zenghui.yu@linux.dev>
Link: https://lore.kernel.org/r/E1s2AxF-00AWLv-03@rmk-PC.armlinux.org.uk


Signed-off-by: Marc Zyngier <maz@kernel.org>