remove ubuntu disco

remove Ubuntu Disco 9.04 as it's EOL - https://en.wikipedia.org/wiki/Ubuntu_version_history#Table_of_versions

roles/netbootxyz/defaults/main.yml

@@ -473,8 +473,6 @@ releases:
     versions:
       - name: "19.10 Eoan Ermine"
         code_name: "eoan"
-      - name: "19.04 Disco Dingo"
-        code_name: "disco"
       - name: "18.04 LTS Bionic Beaver"
         code_name: "bionic"
       - name: "16.04 LTS Xenial Xerus"