perl-modules 5.20.2-6 should not be in our time-based snapshots for Stretch and sid

Originally created by @intrigeri on #11496 (Redmine)

reprepro-time-based-snapshots@apt:~/repositories/debian$ reprepro ls perl-modules
perl-modules | 5.20.2-3+deb8u4 |                  jessie | amd64, i386
perl-modules | 5.20.2-3+deb8u5 | jessie-proposed-updates | amd64, i386
perl-modules |        5.20.2-6 |                 stretch | amd64, i386
perl-modules |        5.20.2-6 |                     sid | amd64, i386

… while this package was removed from Stretch.

Parent Task: tails/sysadmin#5926