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/preferences.py

@@ -225,7 +225,6 @@ class Preferences(PreferencesBase):
         self.scan_tag_genre = get("ScanTagGenre", self.scan_tag_genre)
         self.scan_tag_year = get("ScanTagYear", self.scan_tag_year)
         self.match_scaled = get("MatchScaled", self.match_scaled)
-        self.picture_cache_type = get("PictureCacheType", self.picture_cache_type)
 
     def reset(self):
         self.filter_hardness = 95
@@ -278,7 +277,6 @@ class Preferences(PreferencesBase):
         self.scan_tag_genre = False
         self.scan_tag_year = False
         self.match_scaled = False
-        self.picture_cache_type = "sqlite"
 
     def _save_values(self, settings):
         set_ = self.set_value
@@ -332,7 +330,6 @@ class Preferences(PreferencesBase):
         set_("ScanTagGenre", self.scan_tag_genre)
         set_("ScanTagYear", self.scan_tag_year)
         set_("MatchScaled", self.match_scaled)
-        set_("PictureCacheType", self.picture_cache_type)
 
     # scan_type is special because we save it immediately when we set it.
     def get_scan_type(self, app_mode):