Remove unused serializer from ValidateModelVersion

This serializer has not been used since we removed the ValidateModelVersion endpoint in #1728 (closed)

changed milestone to %Arkindex 1.6.6

requested review from @babadie

assigned to @erouchet

This is detected by vulture

vulture arkindex | grep ModelVersionValidateSerializer
arkindex/training/serializers.py:317: unused class 'ModelVersionValidateSerializer' (60% confidence)

merged