remove old and disused GPU code

[srlantz]

We should excise all the old GPU code, as it hasn't been maintained in a long time and at this point is incompatible with the rest of the code. It's still in the repository if we ever want to go back and look at it.

Most of this code is to be found in separate header and source files. Calls, instantiations, etc. that depend on the GPU-related routines appear in other mkFit source files; however, the affected places are marked with USE_CUDA and similar ifdef's. Also, as pointed out in issue #257, another way to spot the relevant places is to look for commented-out OpenMP directives (other than "omp simd").