Refactor ImportListing

Better to avoid relying on the storyTiddler variable, which is a pain
for people trying to reuse the import listing.

plugins/tiddlywiki/upgrade/UpgradeWizard.tid

@@ -25,12 +25,8 @@ or click to pick a file <$browse/>
 
 The following tiddlers will be included in the upgrade <$button message="tw-perform-import" param="$:/Import">Upgrade</$button>
 
-<$set name="storyTiddler" value="$:/Import">
-
 {{$:/Import||$:/core/ui/ImportListing}}
 
-</$set>
-
 </$reveal>
 
 <$reveal state="$:/Import!!status" type="match" text="complete">