Fix FTBFS with tagged 3.14 snapshots, due to procmail.
procmail is listed with packages said to be “Priority: standard” but it's only “Priority: optional” in stretch. With monkeysphere going away (in 2cee7949), this package is no longer pull and no longer visible in tagged snapshots, which leads to this FTBFS: E: Unable to locate package procmail E: config/chroot_local-hooks/98-remove_unwanted_packages failed (exit non-zero). You should check for errors. Hotfix for 3.14: switch to mutt-style removal, and trigger a purge for procmail only if it's (known and) installed.