Workbench: Remote transmission-cli from unstable.

transmission-cli is not available in testing but right now unstable is
going through the time64_t migration so it's not installable.

workbench/Dockerfile

@@ -144,7 +144,6 @@ RUN apt-get update && \
         tmux \
         tnftp \
         tofu \
-        transmission-cli/unstable \
         tree \
         ttyrec \
         udftools \