Blender Git Loki

Disallow deleting of files by the point cache in the `clear` function.

This function is used in every part of the code, and has been causing
great pain when deleting 15 hours worth of baking in an instant.
The complexity of the code makes it unfeasible to fix this selectively,
so for now simply disallow the code from deleting any of the files.

For selected well-defined cases the deleting can be reenabled later,
e.g. when the user presses the `Bake` button and confirms deletion in
a popup.