KVM: Remove unused kvm_vcpu_gfn_to_pfn_atomic

The last use of kvm_vcpu_gfn_to_pfn_atomic was removed by commit
1bbc60d0 ("KVM: x86/mmu: Remove MMU auditing")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Message-ID: <20241001141354.18009-3-linux@treblig.org>
[Adjust Documentation/virt/kvm/locking.rst. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>