perl: remove _PERL5_DEFAULT_FILE - unnecessary these days?

[rfyu28uyeg_snkmail.com]

Summary

I am unable to reproduce a condition that requires the _PERL5_DEFAULT_FILE in the current ports. This was added in 2015: https://reviews.freebsd.org/D3704.

This change will remove the chance that a stray _PERL5_DEFAULT_FILE can be left in place which can cause problems.

See also comments at https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=227356

Test Plan

Re-install non-default perl port using portmaster. For example, install perl5.24 as the default perl port, remove DEFAULT_VERSIONS=perl5=5.24 from make.conf and use portmaster to re-install perl5.24 (note that perl5.26 is the default now). In my testing, it correctly re-installs perl5.24 as the default perl (with /usr/local/bin/perl, etc.).