Remove enforcement of pressure zero level for pure Neumann problems.

This direct manipulation of the linear system is inappropriate when
solving compressible flows where the absolute pressure level matters
greatly.

src/cfs_tpfa.c

@@ -750,10 +750,6 @@ cfs_tpfa_assemble(grid_t                  *G,
     } else {
         well_is_neumann = 1;
     }
-
-    if (res_is_neumann && well_is_neumann) {
-        h->A->sa[0] *= 2;
-    }
 }