depend-cleanup: remove entries from 2020 and 2021

These tests increase the build time (albeit by a small amount), so
they should be removed once enough time has passed and it is extremely
unlikely anyone would try a NO_CLEAN build against an object tree from
before the related change.

The comment suggests a year is a reasonable period but we'll be somewhat
more conservative for now, in part so that we retain different examples
of special cases.

Reviewed by: brooks, imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46178

(cherry picked from commit 4a06d149371c16287e1dd5a8fa51e92346a0c3f4)