KVM: x86: hyper-v: Drop unused kvm_hv_vapic_assist_page_enabled()

kvm_hv_vapic_assist_page_enabled() seems to be unused since its
introduction in commit 10388a07 ("KVM: Add HYPER-V apic access MSRs"),
drop it.

Reported-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20210126134816.1880136-4-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>