KVM: arm64: Remove unused 'pgd' field from 'struct kvm_s2_mmu'

The stage-2 page-tables are entirely encapsulated by the 'pgt' field of
'struct kvm_s2_mmu', so remove the unused 'pgd' field.

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Quentin Perret <qperret@google.com>
Link: https://lore.kernel.org/r/20200911132529.19844-21-will@kernel.org