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/tests/cache_test.py

@@ -12,7 +12,6 @@ from hscommon.testutil import eq_
 try:
     from core.pe.cache import colors_to_string, string_to_colors
     from core.pe.cache_sqlite import SqliteCache
-    from core.pe.cache_shelve import ShelveCache
 except ImportError:
     skip("Can't import the cache module, probably hasn't been compiled.")
 
@@ -133,11 +132,6 @@ class TestCaseSqliteCache(BaseTestCaseCache):
         eq_(c["foo"], [(1, 2, 3)])
 
 
-class TestCaseShelveCache(BaseTestCaseCache):
-    def get_cache(self, dbname=None):
-        return ShelveCache(dbname)
-
-
 class TestCaseCacheSQLEscape:
     def get_cache(self):
         return SqliteCache()