Fix the creation of public releases

When the release names of consecutive public releases are the identical,
there was a bug:
"gzip: CGAL-4.4-dev.tar.gz already exists;       not overwritten"

Scripts/developer_scripts/create_new_release

@@ -316,6 +316,7 @@ if [ -n "$DO_PUBLIC" ]; then
     # popd
     # cp -a doc_html/* ${public_release_name}/doc_html
 
+    rm -f "${public_release_name}".tar*
     tar -cf "${public_release_name}.tar" "${public_release_name}"
     mkdir "${HTML_DIR}/${release_name}-public"
     xz --best "${public_release_name}.tar" -c > "${HTML_DIR}/${release_name}-public/${public_release_name}.tar.xz"