Remove use of perl parent module in Cluster.pm

Remove use of perl parent module in Cluster.pm

Commit fb16d2c658 used the old but not quite old enough parent module,
which dates to perl version 5.10.1 as a core module. We still have a
dinosaur or two running older versions of perl, so rather than require
an upgrade in those we simply do in place what parent.pm's import()
would have done for us.

Reviewed by Tom Lane

Discussion: https://postgr.es/m/474104.1648685981@sss.pgh.pa.us