KVM: selftests: Remove duplicated TEST_ASSERT in hyperv_cpuid.c

The check for entry->index == 0 is done twice. One time should
be sufficient.

Suggested-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>