Remove obsolete EvtIncoherentMixing class

Summary:
The EvtIncoherentMixing class is no longer used anywhere else in the code,
its functionality having been replaced by EvtCPUtil.

This Differential removes the now obsolete class.

This is associated with !45.

Test Plan: Code compiles and tests give identical results.

Reviewers: abudinen, jback, kreps

Reviewed By: abudinen, jback

Subscribers: #evtgen_admins

Tags: #evtgen

Differential Revision: https://phab.hepforge.org/D137