KVM: x86/mmu: Drop redundant rsvd bits reset for nested NPT

Drop the extra reset of shadow_zero_bits in the nested NPT flow now
that shadow_mmu_init_context computes the correct level for nested NPT.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622175739.3610207-52-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>