Don't remove package-lock.json as it is gone from the repository

pica-etherpad/Dockerfile

@@ -37,7 +37,6 @@ RUN curl -SL https://github.com/ether/etherpad-lite/archive/${ETHERPAD_VERSION_B
     mkdir -p src/templates && \
     bin/installDeps.sh && \
     git clone https://framagit.org/DeBugs/picasoft-etherpad-landing-page.git src/templates/picasoft-etherpad-landing-page && \
-    rm package-lock.json && \
     git --git-dir=src/templates/picasoft-etherpad-landing-page/.git checkout ${ETHERPAD_LANDING_PAGE_VERSION_BUILD} && \
     cp src/templates/picasoft-etherpad-landing-page/index.html src/templates/index.html && \
     cp src/templates/picasoft-etherpad-landing-page/static/* src/static/ -r && \