ci: don't delete the apt cache

Deleting the apt cache can be helpful for reducing the size of a
container, but since we don't push it anywhere, it only hinders our
ability to debug problems while working on the container.  Keep it.

azure-pipelines/docker/xenial

@@ -21,9 +21,7 @@ RUN echo 'deb http://ppa.launchpad.net/hola-launchpad/valgrind/ubuntu xenial mai
         openssl \
         pkgconf \
         python \
-        valgrind \
-        && \
-    rm -rf /var/lib/apt/lists/*
+        valgrind
 RUN mkdir /var/run/sshd
 
 RUN cd /tmp && \