Delete REF_12_conference.py
Delete REF_12_conference.py

No longer used in course.
GoogleCloudPlatformTraining/cpd200-conference-central-python,GoogleCloudPlatformTraining/cpd200-conference-central-python,GoogleCloudPlatformTraining/cpd200-conference-central-python,GoogleCloudPlatformTraining/cpd200-conference-central-python