Drop xenial-support hacks (#10429)

docker/Dockerfile-dhvirtualenv

@@ -15,6 +15,15 @@ ARG distro=""
 ###
 ### Stage 0: build a dh-virtualenv
 ###
+
+# This is only really needed on bionic and focal, since other distributions we
+# care about have a recent version of dh-virtualenv by default. Unfortunately,
+# it looks like focal is going to be with us for a while.
+#
+# (focal doesn't have a dh-virtualenv package at all. There is a PPA at
+# https://launchpad.net/~jyrki-pulliainen/+archive/ubuntu/dh-virtualenv, but
+# it's not obviously easier to use that than to build our own.)
+
 FROM ${distro} as builder
 
 RUN apt-get update -qq -o Acquire::Languages=none
@@ -27,7 +36,7 @@ RUN env DEBIAN_FRONTEND=noninteractive apt-get install \
         wget
 
 # fetch and unpack the package
-# TODO: Upgrade to 1.2.2 once xenial is dropped
+# TODO: Upgrade to 1.2.2 once bionic is dropped (1.2.2 requires debhelper 12; bionic has only 11)
 RUN mkdir /dh-virtualenv
 RUN wget -q -O /dh-virtualenv.tar.gz https://github.com/spotify/dh-virtualenv/archive/ac6e1b1.tar.gz
 RUN tar -xv --strip-components=1 -C /dh-virtualenv -f /dh-virtualenv.tar.gz
@@ -59,8 +68,6 @@ ENV LANG C.UTF-8
 #
 # NB: keep this list in sync with the list of build-deps in debian/control
 # TODO: it would be nice to do that automatically.
-# TODO: Remove the dh-systemd stanza after dropping support for Ubuntu xenial
-#       it's a transitional package on all other, more recent releases
 RUN apt-get update -qq -o Acquire::Languages=none \
     && env DEBIAN_FRONTEND=noninteractive apt-get install \
         -yqq --no-install-recommends -o Dpkg::Options::=--force-unsafe-io \
@@ -76,10 +83,7 @@ RUN apt-get update -qq -o Acquire::Languages=none \
         python3-venv \
         sqlite3 \
         libpq-dev \
-        xmlsec1 \
-    && ( env DEBIAN_FRONTEND=noninteractive apt-get install \
-         -yqq --no-install-recommends -o Dpkg::Options::=--force-unsafe-io \
-         dh-systemd || true )
+        xmlsec1
 
 COPY --from=builder /dh-virtualenv_1.2~dev-1_all.deb /