Revert "reproducible debian: setup_pbuilder: use [check-valid-until=no]...

Revert "reproducible debian: setup_pbuilder: use [check-valid-until=no] instead of Acquire::Check-Valid-Until"

I was checking the wrong log while testing…  it doesn't seem to actually
affect d-i, so let's keep the previous thing.

This reverts commit bbe0f1b2.