Remove libmpdec-dev installation in script

Author: Damien-Chen

.github/workflows/posix-deps-apt.sh

@@ -15,7 +15,6 @@ apt-get -yq install \
     libgdbm-dev \
     libgdbm-compat-dev \
     liblzma-dev \
-    libmpdec-dev \
     libncurses5-dev \
     libreadline6-dev \
     libsqlite3-dev \