Remove Perl_av_top_index

Remove Perl_av_top_index

I created this in 87306e0674dfe3af29804b4641347cd5ac9b0521, thinking it
was needed to preserve backward compatibility if someone were using this
instead of the macro.  But it turned out that there never was such a
function, it was inlined, and the name was S_av_top_index, so there is
no reason to create a new function that no one has ever been able to
call.  So just remove it, and let all accesses go through the macro