Workbench: Temporary remove libprotobuf-dev.

It's currently broken in testing and the version in unstable is part of
the time64_t transition and won't migrate to testing any time soon. I'm
just trying to get the image building again. Probably once the time64_t
transition reaches testing I'll revert this commit.

workbench/Dockerfile

@@ -86,7 +86,6 @@ RUN apt-get update && \
         libnotify-bin \
         libpq-dev \
         libproj-dev \
-        libprotobuf-dev \
         libreadline-dev \
         libsasl2-dev \
         libsecret-tools \