enable ufw firewall again

I had previously awkwardly disabled it because on my VPS the necessary kernel
module wasn't available. I meant to make it an option, but instead I took
out the apt-get install. Now it works for me so I'm putting everything back.

scripts/system.sh

@@ -15,11 +15,9 @@ apt-get -q -y install python3
 
 apt-get install -q -y ntp fail2ban
 
-# Turn on the firewall. First allow incoming SSH, then turn on the firewall. Additional open
-# ports will be set up in the scripts that set up those services. Some virtual machine providers
-# (ehm, Rimuhosting) don't provide a kernel that supports ufw, so let advanced users skip it.
-if [ -z "$DISABLE_FIREWALL" ]; then
-	ufw allow ssh;
-	ufw --force enable;
-fi
+# Turn on the firewall. First allow incoming SSH, then turn on the firewall.
+# Other ports will be opened at the point where we set up those services.
+apt-get -q -y install ufw;
+ufw allow ssh;
+ufw --force enable;