Improve performance when cleaning user categories

src/calibre/library/database2.py

@@ -1196,6 +1196,7 @@ class LibraryDatabase2(LibraryDatabase, SchemaUpgrade, CustomColumns):
             else:
                 new_cats['.'.join(comps)] = user_cats[k]
         try:
+            if new_cats != user_cats:
                 self.prefs.set('user_categories', new_cats)
         except:
             pass