doc: remove duplicated note about UEFI secure boot
A note about secure boot not allowing UIO is present in both the system
requirements section and the driver binding section. This fits better in
the driver binding section, so the copy in system requirements can be
removed. The document in general now also emphasises VFIO over UIO more
than when this note was first added, reducing the need for this warning
to be repeated.
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>