Stop bothering with VMDK 3 --> 2 conversion?

[lentinj]

According to vagrant-mutate, this isn't necessary in QEMU 1.7.0+

• https://github.com/sciurus/vagrant-mutate/wiki/QEMU-Version-Compatibility
• https://github.com/sciurus/vagrant-mutate/issues/28
• https://github.com/sciurus/vagrant-mutate/blob/master/lib/vagrant-mutate/qemu.rb

...can we just get rid of it now?