Removed PolyFit3DParametrizedMagneticField from Threaded branch

The PolyFit3DParametrizedMagneticField is not presently thread safe. To avoid
problems with the static analyzer we are removing it from the threaded
branch. If the code is made thread safe, we will add it back.
Merge pull request #3200 from Dr15Jones/removeMagneticFieldParametrizedEngineFromThreaded

MagneticField/ParametrizedEngine -- Removed PolyFit3DParametrizedMagneticField from Threaded branch