Delete REF_12_ConferenceApi.java
Delete REF_12_ConferenceApi.java

No longer used in course.
sergcourt/gitbox-oneoffs,GoogleCloudPlatformTraining/cpd200-conference-central-java,GoogleCloudPlatformTraining/cpd200-conference-central-java,sergcourt/gitbox-oneoffs,GoogleCloudPlatformTraining/cpd200-conference-central-java,sergcourt/gitbox-oneoffs,sergcourt/gitbox-oneoffs,GoogleCloudPlatformTraining/cpd200-conference-central-java