undeprecate hv_bucket_ratio()

undeprecate hv_bucket_ratio()

This function was recently added, but marked as deprecated in embed.fnc.
Based on this thread:

    http://nntp.perl.org/group/perl.perl5.porters/237486

It looks like the 'M' (may change) flag is sufficient to capture what was
intended. Removing the 'D' avoids every usage in core emitting a warning
to STDERR during build, while it still adds a caution to the entry in
perlapi.pod:

    NOTE: this function is experimental and may change or be
    removed without notice.