deleting DUChain cache dir together with session dir

[Wolfgang Bangerth]

On the topic of this thread: I got into trouble with our system administrators 
because all my students now use KDevelop and they suddenly had these ~1GB 
directories in their home dirs. That's not much of a problem -- unless the GB 
of data is spread over a few ten thousand files and all these are located on 
an NSF mounted home directory located on a file server one building over.

In other words: I can confirm that the fact that this data isn't deleted is a 
problem indeed.

Best
 W.

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Wolfgang Bangerth                email:            bangerth at math.tamu.edu
                                 www: http://www.math.tamu.edu/~bangerth/