feat: Remove shelve picture cache

- Remove shelve picture cache as it has had a fair number of historical
  issues.  Original issue for which it was added should be long
  resolved.  Additionally this allows additional consolidation of the
  various cache code and potentially dbs in the future.
- Remove all related preferences and related code for changing cache
  backend between sqlite and shelve.

qt/app.py

@@ -192,7 +192,6 @@ class DupeGuru(QObject):
             scanned_tags.add("year")
         self.model.options["scanned_tags"] = scanned_tags
         self.model.options["match_scaled"] = self.prefs.match_scaled
-        self.model.options["picture_cache_type"] = self.prefs.picture_cache_type
         self.model.options["include_exists_check"] = self.prefs.include_exists_check
         self.model.options["rehash_ignore_mtime"] = self.prefs.rehash_ignore_mtime