Remove legacy freedns updater from crontab if needed

src/freedombone-utils-dns

     if [ -f /usr/bin/dynamicdns ]; then
         rm /usr/bin/dynamicdns
     fi
+    if grep -q "dynamicdns" /etc/crontab; then
+        sed -i '/dynamicdns/d' /etc/crontab
+    fi
 
     # add the update command to cron
     if ! grep -q "/usr/local/bin/freedombone-freedns" /etc/crontab; then