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.

config/chroot_local-hooks/98-remove_unwanted_packages

@@ -39,7 +39,6 @@ apt-get --yes purge  \
    mlocate \
    ncurses-term \
    nfs-common \
-   procmail \
    python3-reportbug \
    reportbug \
    telnet \
@@ -60,6 +59,12 @@ if is_package_installed mutt; then
     apt-get --yes purge mutt
 fi
 
+### Hotfix for 3.14: procmail is no longer known by apt in tagged
+### snapshots, likely because no longer pulled by monkeysphere:
+if is_package_installed procmail; then
+    apt-get --yes purge procmail
+fi
+
 ### Deinstall some other unwanted packages.
 apt-get --yes purge  \
     '^aptitude*' \