distro/tests: remove obsolete distros

distro/tests/fedora28/Vagrantfile

-# -*- mode: ruby -*-
-# vi: set ft=ruby :
-#
-
-Vagrant.configure(2) do |config|
-
-    config.vm.box = "fedora/28-cloud-base"
-    config.vm.synced_folder ".", "/vagrant", disabled: true
-
-    config.vm.define "fedora28_knot-dns"  do |machine|
-        machine.vm.provision "ansible" do |ansible|
-            ansible.playbook = "../knot-dns-test.yaml"
-            ansible.extra_vars = {
-                ansible_python_interpreter: "/usr/bin/python3"
-            }
-        end
-    end
-
-end

distro/tests/ubuntu1810/Vagrantfile

-# -*- mode: ruby -*-
-# vi: set ft=ruby :
-#
-
-Vagrant.configure(2) do |config|
-
-    config.vm.box = "generic/ubuntu1810"
-    config.vm.synced_folder ".", "/vagrant", disabled: true
-
-    config.vm.define "ubuntu1810_knot-dns"  do |machine|
-        machine.vm.provision "ansible" do |ansible|
-            ansible.playbook = "../knot-dns-test.yaml"
-            ansible.extra_vars = {
-                ansible_python_interpreter: "/usr/bin/python3"
-            }
-        end
-    end
-
-end