Revert "KVM: arm64: Split kvm_pgtable_stage2_destroy()"

This reverts commit 0e89ca13.

The functional change that depended on this refactoring has been found
to be quite problematic. Reverting the whole pile to start fresh when
new fixes are available.

Message-ID: <20250910180930.3679473-3-oliver.upton@linux.dev>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>