Leave icon area blank if legacy note; remove diamond icon

app/helpers/system_note_helper.rb

@@ -37,10 +37,8 @@ module SystemNoteHelper
         'icon_comment_o'
       when 'moved'
         'icon_arrow-circle-o-right'
-      else
-        'icon_diamond'
       end
 
-    custom_icon(icon_name)
+    custom_icon(icon_name) if icon_name
   end
 end

app/views/shared/icons/_icon_diamond.svg

-<svg width="1792" height="1792" viewBox="0 0 1792 1792" xmlns="http://www.w3.org/2000/svg"><path d="M84 768l623 665-300-665h-323zm812 772l349-772h-698zm-486-900l204-384h-262l-288 384h346zm675 793l623-665h-323zm-530-793h682l-204-384h-274zm827 0h346l-288-384h-262zm141-486l384 512q14 18 13 41.5t-17 40.5l-960 1024q-18 20-47 20t-47-20l-960-1024q-16-17-17-40.5t13-41.5l384-512q18-26 51-26h1152q33 0 51 26z"/></svg>