vagrant: remove bionic64 box, fixes #5793

only has py36, below minimum requirement now.

Vagrantfile

@@ -266,18 +266,6 @@ Vagrant.configure(2) do |config|
     b.vm.provision "run tests", :type => :shell, :privileged => false, :inline => run_tests("focal64", ".*none.*")
   end
 
-  config.vm.define "bionic64" do |b|
-    b.vm.box = "ubuntu/bionic64"
-    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("bionic64")
-    b.vm.provision "install borg", :type => :shell, :privileged => false, :inline => install_borg("llfuse")
-    b.vm.provision "run tests", :type => :shell, :privileged => false, :inline => run_tests("bionic64", ".*(fuse3|none).*")
-  end
-
   config.vm.define "bullseye64" do |b|
     b.vm.box = "debian/bullseye64"
     b.vm.provider :virtualbox do |v|