Revert patch 6209, because it made the test fail.

No clue what that patch was supposed to due,
but it even slipped into dune 2.1.

[[Imported from SVN: r6716]]

dune/common/test/fmatrixtest.cc

@@ -225,7 +225,6 @@ void test_matrix()
     FieldVector<K,m> b(1);
 
     A.mv(b, res1);
-    res2 = FMatrixHelp :: mult( A, b );
     A.umv(b, res2);
 
     if( (res1 - res2).two_norm() > 1e-12 )