KVM: Remove unused kvm_vcpu_gfn_to_pfn

The last use of kvm_vcpu_gfn_to_pfn was removed by commit
b1624f99 ("KVM: Remove kvm_vcpu_gfn_to_page() and kvm_vcpu_gpa_to_page()")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Message-ID: <20241001141354.18009-2-linux@treblig.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>