unused miport sys and variable (#13869)

detected not accessed by pylance

Closes #13869

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/13869 from edgarriba:patch-1 177a0c44ab
PiperOrigin-RevId: 573316929

python/google/protobuf/json_format.py

@@ -26,7 +26,6 @@ import json
 import math
 from operator import methodcaller
 import re
-import sys
 
 from google.protobuf.internal import type_checkers
 from google.protobuf import descriptor
@@ -34,7 +33,6 @@ from google.protobuf import message_factory
 from google.protobuf import symbol_database
 
 
-_TIMESTAMPFOMAT = '%Y-%m-%dT%H:%M:%S'
 _INT_TYPES = frozenset([descriptor.FieldDescriptor.CPPTYPE_INT32,
                         descriptor.FieldDescriptor.CPPTYPE_UINT32,
                         descriptor.FieldDescriptor.CPPTYPE_INT64,