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.

core/app.py

@@ -126,8 +126,6 @@ class DupeGuru(Broadcaster):
 
     NAME = PROMPT_NAME = "dupeGuru"
 
-    PICTURE_CACHE_TYPE = "sqlite"  # set to 'shelve' for a ShelveCache
-
     def __init__(self, view, portable=False):
         if view.get_default(DEBUG_MODE_PREFERENCE):
             logging.getLogger().setLevel(logging.DEBUG)
@@ -153,7 +151,6 @@ class DupeGuru(Broadcaster):
             "clean_empty_dirs": False,
             "ignore_hardlink_matches": False,
             "copymove_dest_type": DestType.RELATIVE,
-            "picture_cache_type": self.PICTURE_CACHE_TYPE,
             "include_exists_check": True,
             "rehash_ignore_mtime": False,
         }
@@ -185,8 +182,7 @@ class DupeGuru(Broadcaster):
         self.view.create_results_window()
 
     def _get_picture_cache_path(self):
-        cache_type = self.options["picture_cache_type"]
-        cache_name = "cached_pictures.shelve" if cache_type == "shelve" else "cached_pictures.db"
+        cache_name = "cached_pictures.db"
         return op.join(self.appdata, cache_name)
 
     def _get_dupe_sort_key(self, dupe, get_group, key, delta):