spec: Drop Requires: vbox

Since this package isn't provided by any stock RH based distro. The
upstream RPMs are called VirtualBox anyways.

libvirt.spec.in

@@ -1050,7 +1050,6 @@ Requires: libvirt-daemon-driver-nwfilter = %{version}-%{release}
 Requires: libvirt-daemon-driver-secret = %{version}-%{release}
 Requires: libvirt-daemon-driver-storage = %{version}-%{release}
         %endif
-Requires: vbox
 
 %description daemon-vbox
 Server side daemon and driver required to manage the virtualization