Remove schema registry from pickle for now

It is currently internal, and test fails on non-cython Beam due
  to unpickable proto objects

Author: Abacn

sdks/python/apache_beam/internal/cloudpickle_pickler.py

@@ -197,27 +197,21 @@ def _lock_reducer(obj):
 
 def dump_session(file_path):
   # Since References are saved (https://s.apache.org/beam-picklers), we only
-  # dump known Beam Registries
+  # dump supported Beam Registries (currently only logical type registry)
   from apache_beam.typehints.schemas import LogicalType
-  from apache_beam.typehints import schema_registry
 
   with _pickle_lock, open(file_path, 'wb') as file:
     pickler = cloudpickle.CloudPickler(file)
     # Logical Type registry
     pickler.dump(LogicalType._known_logical_types)
-    # Schema registry
-    pickler.dump(schema_registry.SCHEMA_REGISTRY)
+    # TODO(yathu) consider pickle schema registry, filesystem registry, etc
+    # after these get implemented and/or promoted as public API
 
 
 def load_session(file_path):
   # It is possible to load_session with cloudpickle. However, since references
   # are saved it should not be necessary. See https://s.apache.org/beam-picklers
-  with _pickle_lock, open(file_path, 'rb') as file:
-    from apache_beam.typehints.schemas import LogicalType
-    from apache_beam.typehints import schema_registry
-
-    known_logical_type = cloudpickle.load(file)
-    schema_registry = cloudpickle.load(file)
+  from apache_beam.typehints.schemas import LogicalType
 
-    LogicalType._known_logical_types = known_logical_type
-    schema_registry.SCHEMA_REGISTRY = schema_registry
+  with _pickle_lock, open(file_path, 'rb') as file:
+    LogicalType._known_logical_types = cloudpickle.load(file)