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>

.github/workflows/main.yml

@@ -17,12 +17,6 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - flavor: debian
-            buildsystem: meson
-            tests: unit
-            omemo: signal
-            script: ./scripts/build-configuration-matrix.sh
-            omemo_env: libsignal
           - flavor: fedora
             buildsystem: meson
             tests: unit