qom: remove unused function

qom: remove unused function

The function has been unused since commit 4fa28f23906 ("ppc/pnv:
Instantiate cores separately", 2019-12-17).  The idea was that
you could use it to build an array of objects via pointer
arithmetic, but no one is doing it anymore.

Cc: Dr. David Alan Gilbert <dave@treblig.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>