conf: remove duplicated firmware type attribute

conf: remove duplicated firmware type attribute

The

  <os firmware='efi'>
    <firmware type='efi'>
      <feature enabled='no' name='enrolled-keys'/>
    </firmware>
  </os>

repeats the firmware attribute twice. This has no functional benefit, as
evidenced by fact that we use a single struct field to store both
attributes, while needlessly introducing an error scenario. The XML can
just be simplified to:

  <os firmware='efi'>
    <firmware>
      <feature enabled='no' name='enrolled-keys'/>
    </firmware>
  </os>

which also means that we don't need to emit the empty element
<firmware type='efi'/> for all existing configs too.

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>