KVM: arm64: vgic-its: Get rid of vgic_copy_lpi_list()

The last user has been transitioned to walking the LPI xarray directly.
Cut the wart off, and get rid of the now unneeded lpi_count while doing
so.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240422200158.2606761-7-oliver.upton@linux.dev


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