metainfo: Fix dark screenshot properties

See ximion/appstream#530
We can't have two defaults, and width/height are unnecessary

Author: JohanG-75

data/org.gnome.Sudoku.metainfo.xml.in

@@ -24,11 +24,11 @@
     </p>
   </description>
   <screenshots>
-    <screenshot height="955" width="990" type="default">
+    <screenshot type="default">
       <image>https://gitlab.gnome.org/GNOME/gnome-sudoku/-/raw/master/data/screenshot.png</image>
       <caption>A GNOME sudoku game preview</caption>
     </screenshot>
-    <screenshot height="955" width="990" type="default">
+    <screenshot environment="gnome:dark">
       <image>https://gitlab.gnome.org/GNOME/gnome-sudoku/-/raw/master/data/screenshot-dark.png</image>
       <caption>A GNOME sudoku game in dark mode featuring earmarks and warnings</caption>
     </screenshot>