Remove adolctest-scalar-and-vector-mode.cc

ADOL-C can compute all derivatives with two different modes:
scalar- and vector mode.  In theory they should produce exactly
the same results, but sometimes there are bugs (I have found
a few of those myself).  adolctest-scalar-and-vector-mode tested
whether the two modes produced the same result for a particular
Cosserat configuration.

I do not think that dune-gfe is the right place for such a test.
It tests for bugs in ADOL-C itself, and that is the job of
ADOL-C's own test suite.  Consequently, I am removing the test.