MPI: Remove superflous check on environment variables.

The fact that the environment exists already is sufficient to tell
whether or not a program using an ACQt FileManager is run with mpirun.

This is the way it's been done everywhere else in the code, so follow
the convention.
ParticleFactoryBase: Remove warning when trying to create SplineTrack.

If a particle does not have an associated tracker track (or track fit)
the function should just silently exit instead of spamming the user with
WARN_OUT messages. After all, it's normal that some particles do not
have an associated tracker track.