vagrant: remove the xenial box

this box now runs into pip vs py35 compatibility issues,
the pip that gets installed uses py36 features and crashes
with SyntaxError.

considering that xenial is running out of support 2021/04
anyway, i am just removing this box rather than fixing this.

borg is still tested with py35 via github actions / via pyenv.

Vagrantfile

@@ -381,18 +381,6 @@ Vagrant.configure(2) do |config|
     b.vm.provision "run tests", :type => :shell, :privileged => false, :inline => run_tests("bionic64")
   end
 
-  config.vm.define "xenial64" do |b|
-    b.vm.box = "ubuntu/xenial64"
-    b.vm.provider :virtualbox do |v|
-      v.memory = 1024 + $wmem
-    end
-    b.vm.provision "fs init", :type => :shell, :inline => fs_init("vagrant")
-    b.vm.provision "packages debianoid", :type => :shell, :inline => packages_debianoid("vagrant")
-    b.vm.provision "build env", :type => :shell, :privileged => false, :inline => build_sys_venv("xenial64")
-    b.vm.provision "install borg", :type => :shell, :privileged => false, :inline => install_borg(true)
-    b.vm.provision "run tests", :type => :shell, :privileged => false, :inline => run_tests("xenial64")
-  end
-
   config.vm.define "buster64" do |b|
     b.vm.box = "debian/buster64"
     b.vm.provider :virtualbox do |v|