KVM: arm64: Remove unused __sve_save_state

Now that we don't have any users left for __sve_save_state, remove
it altogether. Should we ever need to save the SVE state from the
hypervisor again, we can always re-introduce it.

Suggested-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>