Remove 'mercurial', it's no longer needed

It was used in the past to clone Coin and SoQt when they were hosted on BitBucket instead of GitHub.

Remove 'mercurial', it's no longer needed
4f4dc35a · Riccardo Maria Bianchi · 67eccb48 · 4f4dc35a
Commit 4f4dc35a authored 4 months ago by Riccardo Maria Bianchi
--- a/Dockerfile
+++ b/Dockerfile
@@ -24,7 +24,6 @@ RUN apt-get update -qq && \
    unzip \
    build-essential \
    freeglut3-dev \
-    mercurial \
    nlohmann-json3-dev \
    libexpat1-dev \
    apt-utils \