vbox: Remove unused mediumEmpty
vbox: Remove unused mediumEmpty

Remove the setting since it's unused as of commit 34364df3 which should
have never copied it in from the old code which ended up getting removed
as part of commit c7c286c6.

fabianfreyer/libvirt,jfehlig/libvirt,eskultety/libvirt,jardasgit/libvirt,jardasgit/libvirt,jardasgit/libvirt,crobinso/libvirt,jardasgit/libvirt,jfehlig/libvirt,nertpinx/libvirt,andreabolognani/libvirt,olafhering/libvirt,eskultety/libvirt,zippy2/libvirt,nertpinx/libvirt,crobinso/libvirt,olafhering/libvirt,libvirt/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,zippy2/libvirt,eskultety/libvirt,libvirt/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,crobinso/libvirt,eskultety/libvirt,libvirt/libvirt,jfehlig/libvirt,jardasgit/libvirt,olafhering/libvirt,andreabolognani/libvirt,zippy2/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,olafhering/libvirt,zippy2/libvirt,eskultety/libvirt,libvirt/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,crobinso/libvirt,jfehlig/libvirt