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/pe/matchblock.py

@@ -16,6 +16,7 @@ from hscommon.jobprogress import job
 
 from core.engine import Match
 from core.pe.block import avgdiff, DifferentBlockCountError, NoBlocksError
+from core.pe.cache_sqlite import SqliteCache
 
 # OPTIMIZATION NOTES:
 # The bottleneck of the matching phase is CPU, which is why we use multiprocessing. However, another
@@ -50,14 +51,7 @@ except Exception:
 
 
 def get_cache(cache_path, readonly=False):
-    if cache_path.endswith("shelve"):
-        from core.pe.cache_shelve import ShelveCache
-
-        return ShelveCache(cache_path, readonly=readonly)
-    else:
-        from core.pe.cache_sqlite import SqliteCache
-
-        return SqliteCache(cache_path, readonly=readonly)
+    return SqliteCache(cache_path, readonly=readonly)
 
 
 def prepare_pictures(pictures, cache_path, with_dimensions, j=job.nulljob):