ci: Let Debian build only with libomemo-c

So we can test if it works if libsignal isn't even installed on the
system.

Ref: 9a501e6ecd
Signed-off-by: Michael Vetter <jubalh@iodoru.org>

ci/Dockerfile.debian

@@ -20,7 +20,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   libnotify-dev \
   libotr5-dev \
   libreadline-dev \
-  libsignal-protocol-c-dev \
   libomemo-c-dev \
   libssl-dev \
   libtool \