Simplified pre-processor behaviour for PHOTOS.

Summary:
Moved pre-processor ifdef conditions from EvtPHOTOS class to EvtExternalGenList.

This is associated with T250 and !17.

Test Plan: Compilation and running with PHOTOS works.

Reviewers: tlatham, jback, kreps

Reviewed By: tlatham

Tags: #evtgen

Differential Revision: https://phab.hepforge.org/D114