KVM: arm64: Drop long gone function parameter documentation

update_vmid() just has one parameter "vmid".The other parameter
"kvm" is no longer used.

Signed-off-by: Peng Hao <richard.peng@oppo.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200701120709.388377-1-richard.peng@oppo.com