Do not check for zero entries when restricting occupation patterns.

Because some people restrict patterns for uninitialized matrices.
In that case you get
a) valgrind warnings
b) crashes, if an entry happens to be zero, it is not restricted.

[[Imported from SVN: r6332]]

dune/solvers/transferoperators/genericmultigridtransfer.hh

@@ -770,8 +770,6 @@ public:
 
             for (; m!=mEnd; ++m)
             {
-                if (m->infinity_norm()==0)
-                    continue;
                 int w = m.index();
 
                 // Loop over all coarse grid vectors iv that have v in their support
@@ -831,8 +829,6 @@ public:
 
             for (; m!=mEnd; ++m)
             {
-                if (m->infinity_norm()==0)
-                    continue;
                 int w = m.index();
                 int w_block = w/virtualBlockSize;
                 int w_inblock = w%virtualBlockSize;