Remove the file 'example.cc'

This file, while being called 'example', was actually a set of unit tests. The stuff it tested was all covered by the standard unit tests. Having these additional tests in a file with a misleading name, and at a location outside of the standard test locations, was confusing (for me, at least). This patch simply removes the file.

With the 'example.cc' file gone, the directory 'dune/istl/tutorial' is empty. Therefore this patch removes that directory, too.