schemas: backup: Remove pointless <choice> for 'name' of backup disk

One of the first versions thought of using disk path as the second
option but this was dropped as being a legacy interface. Remove the
leftover pointless <choice> wrapper for the disk name as there's just
one option now.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

docs/schemas/domainbackup.rng

@@ -104,9 +104,7 @@
         <oneOrMore>
           <element name='disk'>
             <attribute name='name'>
-              <choice>
-                <ref name='diskTarget'/>
-              </choice>
+              <ref name='diskTarget'/>
             </attribute>
             <choice>
               <group>
@@ -165,9 +163,7 @@
         <oneOrMore>
           <element name='disk'>
             <attribute name='name'>
-              <choice>
-                <ref name='diskTarget'/>
-              </choice>
+              <ref name='diskTarget'/>
             </attribute>
             <choice>
               <group>