Remove VTK_USE_LARGE_DATA option (turn always on)

I would like to see the VTK_USE_LARGE_DATA option removed from the VTK CMake with the behavior changed to always use the "large data," which is not really very large.

This option was originally created because we had some tests that required reading in data on the order of 10MB to 100MB. Although these data sets are not particularly large, at the time all VTK data files for testing were stored in a CVS repository and we were worried that adding even moderately sized files could strain the revision system originally designed for source files less than 1 MB. To resolve the issue, we created a separate repository for the "large" files that would have many fewer revisions than the main data repository.

Now that we have moved away from storing data files as external data, the point of this option is moot. There is no longer any practical reason why anyone who has already committed to downloading the ~400MB of "small" data files wouldn't be able to download the extra ~400MB of "large" data files. This option now just serves to turn off some valuable VTK testing. So let's just get rid of VTK_USE_LARGE_DATA and always enable those tests.