remove unused libzstd-dev dependency from x64 linux test runner

Author: lqd

src/ci/docker/host-x86_64/x86_64-gnu/Dockerfile

@@ -18,7 +18,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   xz-utils \
   mingw-w64 \
   zlib1g-dev \
-  libzstd-dev \
   && rm -rf /var/lib/apt/lists/*
 
 COPY scripts/sccache.sh /scripts/