Remove unused _quicksort sub-pragma, no longer supported in Perl 5.27

Bug-Debian: https://bugs.debian.org/898989
Author: Dominic Hargreaves <dom@earth.li>

ALTree/SitePerForet.pm

@@ -7,7 +7,6 @@ package ALTree::SitePerForet;
 ################################################################
 
 use base qw(ALTree::Base ALTree::Site);
-use sort '_quicksort';
 
 sub New { # [classe] site_nb
     my $class=shift;