Remove P2G Support for CSPP Active Fire .txt files

[kathys]

Please remove support (recognizing) the CSPP Active Fire .txt files as input to the P2G viirs_edr_active_fires reader. Since these output .txt files are only produced by the CSPP Active Fire software when there is at least one thermal anomaly found in a granule, there were many times when the overpass output files were non-contiguous and would cause P2G to fail.

The software works well on the CSPP Active Fire output NetCDF files, which are produced for all input granules, so that is what will remained supported.

[djhoese]

Right now we are pretty blindly passing the files to the Satpy Scene object. I just looked at the code and it might not be very straight forward to have Polar2Grid say "I see you passed these files, we don't like these files for this particular reader" when that logic isn't coming from Satpy. But that's future Dave's problem.