Remove an incorrect assertion.

The statement is only correct during the very first grid pass whilst
constructing the matrix.  Source terms are added later, in arbitrary
order, and we cannot know (in assembleBlock) what (block) rows are
affected by such contributions.

src/CSRMatrixBlockAssembler.hpp

@@ -61,7 +61,6 @@ namespace Opm {
 
                 assert (ndof >  0);
                 assert (ndof == ndof_);
-                assert (i    == (ia_.size() - 1 - 1) / ndof_);
 
                 const ::std::size_t start = ia_[i*ndof + 0];
                 const ::std::size_t off   =