[Perl/perl5] e4543a: hv.c: Remove obsolete/confusing constant

[Eric Herman]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: e4543a2055aeeb5ef42eb7a84fae20b71643c972
      
https://github.com/Perl/perl5/commit/e4543a2055aeeb5ef42eb7a84fae20b71643c972
  Author: Eric Herman <e...@freesa.org>
  Date:   2020-07-10 (Fri, 10 Jul 2020)

  Changed paths:
    M hv.c

  Log Message:
  -----------
  hv.c: Remove obsolete/confusing constant

The HV_FILL_THRESHOLD is no longer used and is misleading.

See also:
commit 8bf4c4010cc474d4000c2a8c78f6890fa5f1e577
Date:   Mon Jun 20 22:51:38 2016 +0200
        Change scalar(%hash) to be the same as 0+keys(%hash)