Remove redundant perl version checks

Remove redundant perl version checks

Commit 4c1532763a removed some redundant uses of 'use 5.008001;' in perl
scripts, including in plperl's plc_perlboot.pl. Because it made other
changes it wasn't backpatched. However, now this is causing a failure on
back branches when built with bleeding edge perl. Therefore, backpatch
just that part of it which removed those uses, from 15 all the way down
to 9.2, which is the earliest version currently built in the buildfarm.

per report from Alexander Lakhin

Discussion: http://postgr.es/m/4cc2ee93-e03c-8e13-61ed-412e7e6ff19d@gmail.com