Remove obsolete ISR flag.

Summary:
Removed obsolete ISR flag.
Set FSR flag by default to 0 for photons.

This is related with T250 and !21.

Test Plan: Run tests with FSR. Results do not change.

Reviewers: kreps, tlatham, jback

Reviewed By: tlatham

Subscribers: #evtgen_admins

Tags: #evtgen

Differential Revision: https://phab.hepforge.org/D118