remove calculation of good channels on data load

[shashwatsridhar]

The calculation good channels takes place during data loading in the current implementation of pykilosort. This is done differently in the matlab version, wherein the good channels are calculated as a part of the preprocessing step and the probe view is updated thereafter. This should be the default behaviour in pykilosort as well, and would speed up data loading significantly.

Note: this is necessary to mimic the functionality of Kilosort2, but the calculation of good channels is likely to be redundant in the upcoming datashift version of Kilosort / Pykilosort. @marius10p can correct me if I'm wrong here