Remove bfield and band structure code from GPU avalanche (single particle)

I don't really know why, but in 0f7a7b2e the TransportElectron method of AvalancheMicroscopic was split into three cases for regular, with bfield, and semi-conductor versions. Since we haven't implemented the GPU code for the bfield and semi-conductor versions anyway it works out well because we can safely remove commented out code from our version.

Does this seem ok to you @mslater? My thinking is the difference with other commented code in that method is that we might want to implement some of the other functionality at some point